She needs to file a report with the local PD or SO. They may not be able to arrest him for anything so far (like harassing communications), but having the report filed will help if things escalate. I would think that someone on here would be tech savvy enough to explain how the guy is able to send messages from her number without using her phone. But, a law enforcement agency with a cyber crimes specialist could probably explain how and what can be done to prevent it. Does she have children with the guy? If she is going to have to continue to deal with him because of children together, things may get a lot messier.