<human>: Give me the gist of the text in a nutshell

My girlfriend and I started dating about 4 months ago. So far it's being going really well. However, our (formerly) close friend also had feeling for her and is extremely jealous. I posted a [question] asking for help in /r/relationship_advice/ and followed it. We have now cut contact from him. 

In his efforts of trying to get with my girlfriend however, he made up rumours about my girlfriend trying to sabotage our relationship. It would include how she sent sexually suggestive messages to attract my attention and how she manipulated me into dating her. 

My girlfriend has hung out with the same group of friends for almost 12 years now. They have always been extremely close and would trust each other for everything. This time though, they decided to trust our friend instead.  

Her friends are now excluding her from their social circle and calling her derogatory terms, e.g. slut, whore, etc. My girlfriend feels extremely betrayed. We're trying to find a way to minimize the damages and if possible get them along again (although IMO they don't seem like the best of friends, but then again they've been together for 12 years). She says she is willing to give them up if it came to that. Unfortunately, there aren't that many female social groups in our school. Theirs is one of the largest. She would not get along with the other groups. I've suggested hanging out with my friends and me. 

How would we go about solving this?
<bot>: friend is jealous and starts rumours about how gf manipulated me into dating her; gf's friends of 12 years believes him and thinks gf is a bad person; what's the best course of action?