This is a very trickey situation and we went through this nonsense with my daughter. Here is the catch. He bought and contracted the phone for her. She never signed the contract?? If not, then he is soley responsble for the phone and the account. In all fairness, your daughter should remove all personal information like messages, and phone numbers from the phone and return it to him. Do not keep the phone and use it. In the case of my daughter, the jerk defaulted on paying the bill for the phone, but he argued that he bought the phone as a gift for her and she had agreed to pay for it. This of course is crap. They have nothing in writing between them. However, the bone heads at the phone company and their collection agencies, still on occasion attempt to collect from her. We return all correspondence with a request in writing to see a signed contract. This is not possible and usually stops them in their tracks for a few months. If your daughter did not sign a contract either with the phone company, or her ex-boyfriend, neither has any recourse against her as its all hearsay from that point on and would not hold up in court. Her credit is safe as well. At that point, you just take her to a phone store and get her, her own phone and plan.
Now, if she did co-sign the contract for the phone with the ex-boyfriend, then you should be able to take this up with the phone company. This is where is gets complicated. He is obviously the primary on the account, and seeing that there are more than one phone its seems like its possibly a family plan. I'm not sure if a secondary on the account can do anything without the primary being there outside of buying a new phone for the same plan. As I said, you need to call Verizon or AT&T and talk to them to see what their policies are, but only do that if she signed the agreement with him.
What your hoping to do is have them agree to drop her phone from his account and re-open it in an account of her own. This can be done, but she would have to agree to like a new 1 year or 2 year contract. So a lot depends on if she signed anything. The easiest route is if she did not, then she cannot be held liable for the phone he bought and let her use, and she can just return the phone to him and go out and get her own. If she did sign you need to talk to the phone service and see if she can do anything as a secondary on the account, and if so good, close her secondary and flip it to her own account. If not, then she is at the mercy of him paying for the phone each month until the contract ends. If he stops paying, she can be held liable and would have to make sure she stopped in at the company each month and pay her part of the bill. That gets real messy though especially if he had opened a family plan. Good luck.