theorem denseRange_coeSimpleFuncNonnegToLpNonneg [hp : Fact (1 ≤ p)] (hp_ne_top : p ≠ ∞) :
    DenseRange (coeSimpleFuncNonnegToLpNonneg p μ G) := fun g ↦ by
  borelize G
  rw [mem_closure_iff_seq_limit]
  have hg_memℒp : Memℒp (g : α → G) p μ := Lp.memℒp (g : Lp G p μ)
  have zero_mem : (0 : G) ∈ (range (g : α → G) ∪ {0} : Set G) ∩ { y | 0 ≤ y } := by
    simp only [union_singleton, mem_inter_iff, mem_insert_iff, eq_self_iff_true, true_or_iff,
      mem_setOf_eq, le_refl, and_self_iff]
  have : SeparableSpace ((range (g : α → G) ∪ {0}) ∩ { y | 0 ≤ y } : Set G) := by
    apply IsSeparable.separableSpace
    apply IsSeparable.mono _ (Set.inter_subset_left _ _)
    exact
      (Lp.stronglyMeasurable (g : Lp G p μ)).isSeparable_range.union
        (finite_singleton _).isSeparable
  have g_meas : Measurable (g : α → G) := (Lp.stronglyMeasurable (g : Lp G p μ)).measurable
  let x n := SimpleFunc.approxOn g g_meas ((range (g : α → G) ∪ {0}) ∩ { y | 0 ≤ y }) 0 zero_mem n
  have hx_nonneg : ∀ n, 0 ≤ x n := by
    intro n a
    change x n a ∈ { y : G | 0 ≤ y }
    have A : (range (g : α → G) ∪ {0} : Set G) ∩ { y | 0 ≤ y } ⊆ { y | 0 ≤ y } :=
      inter_subset_right _ _
    apply A
    exact SimpleFunc.approxOn_mem g_meas _ n a
  have hx_memℒp : ∀ n, Memℒp (x n) p μ :=
    SimpleFunc.memℒp_approxOn _ hg_memℒp _ ⟨aestronglyMeasurable_const, by simp⟩
  have h_toLp := fun n => Memℒp.coeFn_toLp (hx_memℒp n)
  have hx_nonneg_Lp : ∀ n, 0 ≤ toLp (x n) (hx_memℒp n) := by
    intro n
    rw [← Lp.simpleFunc.coeFn_le, Lp.simpleFunc.toLp_eq_toLp]
    filter_upwards [Lp.simpleFunc.coeFn_zero p μ G, h_toLp n] with a ha0 ha_toLp
    rw [ha0, ha_toLp]
    exact hx_nonneg n a
  have hx_tendsto :
      Tendsto (fun n : ℕ => snorm ((x n : α → G) - (g : α → G)) p μ) atTop (𝓝 0) := by
    apply SimpleFunc.tendsto_approxOn_Lp_snorm g_meas zero_mem hp_ne_top
    · have hg_nonneg : (0 : α → G) ≤ᵐ[μ] g := (Lp.coeFn_nonneg _).mpr g.2
      refine' hg_nonneg.mono fun a ha => subset_closure _
      simpa using ha
    · simp_rw [sub_zero]; exact hg_memℒp.snorm_lt_top
  

Proof state:
α : Type u_1
β : Type u_2
ι : Type u_3
E : Type u_4
F : Type u_5
𝕜 : Type u_6
inst✝³ : MeasurableSpace α
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedAddCommGroup F
p : ℝ≥0∞
μ : Measure α
G : Type u_7
inst✝ : NormedLatticeAddCommGroup G
hp : Fact (1 ≤ p)
hp_ne_top : p ≠ ⊤
g : { g // 0 ≤ g }
this✝¹ : MeasurableSpace G := borel G
this✝ : BorelSpace G
hg_memℒp : Memℒp (↑↑↑g) p
zero_mem : 0 ∈ (Set.range ↑↑↑g ∪ {0}) ∩ {y | 0 ≤ y}
this : SeparableSpace ↑((Set.range ↑↑↑g ∪ {0}) ∩ {y | 0 ≤ y})
g_meas : Measurable ↑↑↑g
x : ℕ → α →ₛ G := fun n => SimpleFunc.approxOn (↑↑↑g) g_meas ((Set.range ↑↑↑g ∪ {0}) ∩ {y | 0 ≤ y}) 0 zero_mem n
hx_nonneg : ∀ (n : ℕ), 0 ≤ x n
hx_memℒp : ∀ (n : ℕ), Memℒp (↑(x n)) p
h_toLp : ∀ (n : ℕ), ↑↑(Memℒp.toLp ↑(x n) (_ : Memℒp (↑(x n)) p)) =ᵐ[μ] ↑(x n)
hx_nonneg_Lp : ∀ (n : ℕ), 0 ≤ toLp (x n) (_ : Memℒp (↑(x n)) p)
hx_tendsto : Tendsto (fun n => snorm (↑(x n) - ↑↑↑g) p μ) atTop (𝓝 0)
⊢ ∃ x, (∀ (n : ℕ), x n ∈ Set.range (coeSimpleFuncNonnegToLpNonneg p μ G)) ∧ Tendsto x atTop (𝓝 g)

Next tactic:
refine'
    ⟨fun n =>
      (coeSimpleFuncNonnegToLpNonneg p μ G) ⟨toLp (x n) (hx_memℒp n), hx_nonneg_Lp n⟩,
      fun n => mem_range_self _, _⟩