/-- For any `S`-module Y, there is a natural `R`-linear map from `S ⨂ Y` to `Y` by
`s ⊗ y ↦ s • y`
-/
@[simps apply]
def Counit.map {Y} : (restrictScalars f ⋙ extendScalars f).obj Y ⟶ Y := by
  letI m1 : Module R S := Module.compHom S f
  letI m2 : Module R Y := Module.compHom Y f
  refine'
    {toFun := TensorProduct.lift
      {toFun := fun s : S => {toFun := fun y : Y => s • y, map_add' := smul_add _, map_smul' := _},
        map_add' := _, map_smul' := _}, map_add' := _, map_smul' := _}
  · intros r y
    dsimp
    change s • f r • y = f r • s • y
    rw [← mul_smul, mul_comm, mul_smul]
  · intros s₁ s₂
    ext y
    change (s₁ + s₂) • y = s₁ • y + s₂ • y
    rw [add_smul]
  · intros r s
    

Proof state:
case refine'_3
R✝ : Type u₁
S✝ : Type u₂
inst✝³ : CommRing R✝
inst✝² : CommRing S✝
f✝ : R✝ →+* S✝
R : Type u₁
S : Type u₂
inst✝¹ : CommRing R
inst✝ : CommRing S
f : R →+* S
Y : ModuleCat S
m1 : Module R S := Module.compHom S f
m2 : Module R ↑Y := Module.compHom (↑Y) f
r : R
s : S
⊢ AddHom.toFun
      {
        toFun := fun s =>
          { toAddHom := { toFun := fun y => s • y, map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) },
            map_smul' :=
              (_ :
                ∀ (r : R) (y : ↑Y),
                  AddHom.toFun
                      { toFun := fun y => s • y, map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                      (r • y) =
                    (RingHom.id R) r •
                      AddHom.toFun
                        { toFun := fun y => s • y, map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                        y) },
        map_add' :=
          (_ :
            ∀ (s₁ s₂ : S),
              (fun s =>
                    {
                      toAddHom :=
                        { toFun := fun y => s • y, map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) },
                      map_smul' :=
                        (_ :
                          ∀ (r : R) (y : ↑Y),
                            AddHom.toFun
                                { toFun := fun y => s • y,
                                  map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                (r • y) =
                              (RingHom.id R) r •
                                AddHom.toFun
                                  { toFun := fun y => s • y,
                                    map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                  y) })
                  (s₁ + s₂) =
                (fun s =>
                      {
                        toAddHom :=
                          { toFun := fun y => s • y,
                            map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) },
                        map_smul' :=
                          (_ :
                            ∀ (r : R) (y : ↑Y),
                              AddHom.toFun
                                  { toFun := fun y => s • y,
                                    map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                  (r • y) =
                                (RingHom.id R) r •
                                  AddHom.toFun
                                    { toFun := fun y => s • y,
                                      map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                    y) })
                    s₁ +
                  (fun s =>
                      {
                        toAddHom :=
                          { toFun := fun y => s • y,
                            map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) },
                        map_smul' :=
                          (_ :
                            ∀ (r : R) (y : ↑Y),
                              AddHom.toFun
                                  { toFun := fun y => s • y,
                                    map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                  (r • y) =
                                (RingHom.id R) r •
                                  AddHom.toFun
                                    { toFun := fun y => s • y,
                                      map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                    y) })
                    s₂) }
      (r • s) =
    (RingHom.id R) r •
      AddHom.toFun
        {
          toFun := fun s =>
            {
              toAddHom :=
                { toFun := fun y => s • y, map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) },
              map_smul' :=
                (_ :
                  ∀ (r : R) (y : ↑Y),
                    AddHom.toFun
                        { toFun := fun y => s • y, map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                        (r • y) =
                      (RingHom.id R) r •
                        AddHom.toFun
                          { toFun := fun y => s • y, map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                          y) },
          map_add' :=
            (_ :
              ∀ (s₁ s₂ : S),
                (fun s =>
                      {
                        toAddHom :=
                          { toFun := fun y => s • y,
                            map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) },
                        map_smul' :=
                          (_ :
                            ∀ (r : R) (y : ↑Y),
                              AddHom.toFun
                                  { toFun := fun y => s • y,
                                    map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                  (r • y) =
                                (RingHom.id R) r •
                                  AddHom.toFun
                                    { toFun := fun y => s • y,
                                      map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                    y) })
                    (s₁ + s₂) =
                  (fun s =>
                        {
                          toAddHom :=
                            { toFun := fun y => s • y,
                              map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) },
                          map_smul' :=
                            (_ :
                              ∀ (r : R) (y : ↑Y),
                                AddHom.toFun
                                    { toFun := fun y => s • y,
                                      map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                    (r • y) =
                                  (RingHom.id R) r •
                                    AddHom.toFun
                                      { toFun := fun y => s • y,
                                        map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                      y) })
                      s₁ +
                    (fun s =>
                        {
                          toAddHom :=
                            { toFun := fun y => s • y,
                              map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) },
                          map_smul' :=
                            (_ :
                              ∀ (r : R) (y : ↑Y),
                                AddHom.toFun
                                    { toFun := fun y => s • y,
                                      map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                    (r • y) =
                                  (RingHom.id R) r •
                                    AddHom.toFun
                                      { toFun := fun y => s • y,
                                        map_add' := (_ : ∀ (b₁ b₂ : ↑Y), s • (b₁ + b₂) = s • b₁ + s • b₂) }
                                      y) })
                      s₂) }
        s

Next tactic:
ext y