Please help!

Hello everyone, I would like to tell you this is my first post on eve and I'm kind of stuck and need help. A girl recently started liking my boyfriend. She bothers him at work and constantly messaging him on facebook and tagging him in posts. He has tried many times to tell her he is not interested and has a girlfriend. She won't stop and he suspects she might have a mental disability. He's trying to be polite about it but it's getting to the point where he is starting to snap at her and he is close to filing a harassment charge on her. He asked me to intervene as I told him I would of this wasn't solved through his methods. I messaged her on facebook explaining who I was and that she needs to stop, if not matters will be taken legally. She has not stopped and I don't know what to do! I live an hour away from my boyfriend so confronting her in person is not always an option. Please help me and guide me on how to make her stop. Any suggestions are welcome. Thank you and I apologize for the long message :)