What's the best way to keep a scammer from contacting u on yahoo messenger if they have ur cell number?

I have a friend who was talkin to a guy who she descovered was a scammer. By that time she had already given him her cell number. He contacts her thru yahoo messenger and just sends the messages straight to her phone. Question I have is ;is there a way to block him from her number , or will she have to get a new number. Even if she gets a new number will it automatically show on her yahoo info and he be able to get her number from that