The first thing that came to my mind---how about asking her to change her Cell Phone number? That way the ex can't get in touch with her. If she is willing to do this and otherwise behaves in a trustworthy manner, then that should settle the problem.
However, if she absolutely refuses to change her number, ask her why. If she says that all her friends/family have the old number, tell her it's easy enough to get in touch with those who you want to have the number and let them know. For example, since our daughter has been on her own, she has changed her Cell Phone number a few times. Each time she was able to let everyone know.