If a couple does fight physically, try to intervene lol. But I don't allow my girlfriend to talk trash about my friends unless I know it is valid. Sometimes they are looking out for you and you have to accept that you got bum friends. I'm assuming you have good judgment though.

Also, if your friend's girlfriend is talking trash about the friend group, you can't say much or give an input. Most you can do is say it's not right and that's up to your friend. Best solution is to not make it our problem.