Hi, I'd like to help you with your question.
It sounds like your daughter's boyfriend is at the least emotionally abusive. I know you mentioned that you did not feel he was abusive to her, but from your description, his behavior indicates that he is manipulative and controlling.
Abusive relationships can happen to anyone. Most abusers are very charming and easily disarm most people. Unless you are looking for it, you usually don't see it, even the person in the relationship. And it sounds like that is what happened to your daughter.
Your daughter's boyfriend is playing a game with her. He is trying to see if he can control her. No one "takes a break" from a relationship then proceeds to tell their partner what they can and cannot do. His mean texts and checking up on her is also an indication that he is trying to control her. If he truly needed a break, he would give her space and if he contacted her, he would be nice to her. He would also be concerned with how she is reacting to the "break". If she is upset enough to be on medication, then he should be worried about that. Not about who she is with and where she is.
The best thing your daughter can do is end the relationship. It is very painful, but it also gives her some control over her own feelings. She can mourn the relationship and move on. You can help her by learning more about abusive relationships and ways she can identify what is happening and how to get out. Here are some resources to help:
The Emotionally Destructive Relationship: Seeing It, Stopping It, Surviving It by Leslie Vernick
The Emotionally Abused Woman : Overcoming Destructive Patterns and Reclaiming Yourself by Beverly Engel
You can find these books on Amazon.com or your local library may have them for you.
I hope this has helped you and your daughter,