but I will not be the guy that tries to isolate his GF from her friends

Setting boundaries in a relationship is not at all the same as isolating someone from their friends or family. Way too much pressure is put on men to not be "toxic" or "controlling" to the point that men allow women to walk all over their relationship until they start cheating, specifically because of things like this.

This is a great opportunity to see where you stand. You set a boundary and if it's not respected, you find a new relationship.

She shouldn't be hanging out with a toxic guy like that who she has a sexual past with, who is known to sleep with peoples girlfriends. She shouldn't want to all on her own.