The hearts in your friend menu tell you about the status of your mutual visits:
Empty Heart = You haven't helped your friend today, and they haven't helped you.
Left Half Red = You have helped your friend, but they haven't helped you.
Right Half Red = Your friend helped you, but you didn't help them.
All Red = You helped each other.
Locked Heart = You just became friends and need to wait another day before you can help them.
Just be patient and the black heart will go away and you'll be able to visit your friend!