Make things short:

My girlfriends best girl friend asked her to join a threesome with her boyfriend.

She obviously said no that's why she told me about it. Problem here is her friend and her boyfriend. Once he even told her in a club that he wants to fuck her (he fucks girls all the time his GF doesn't mind). That time I kind off let it slide off since he apologized next day saying he was really drunk etc.

However this time he reached the limit and I'm thinking what action to take:

1. Shoot them both.
2. Tell both of them to fuck off from my GF.

Problem with 2nd option is that I rarely see them (almost never) specially when my GF is not around. And I don't want to do it over the phone since I don't think it would be effective.

Any suggestions? Beat the guy up? Talk with him? Talk calmly, aggressive? Problem is they might tell my GF about this so I do not want to appear insecure etc. and make things worse.

First, never fight a guy over a chick. Unless it's to protect her and she can't.

Your gf is a grown woman; if she let's a guy disrespect your relationship, that's on her.

Only get involved with a girl's problems when she has taken all the steps she can to fix the situation. You fight him, she's still gonna hang out with them and then you look stupid

Don't fight him unless it has to come to it. In person just tell him and her to fuck right off. If your girlfriend has a problem with you being so upfront about it then show her the door and never look back -- she isn't worth it in that case.

Seriously, that shit is out of line by any stretch of the imagination. Sometimes you have to stand up on your own two legs and be ready to beat the crap out of someone even if you need to get some friends together.