Backstory: >Have been dating a girl secretly for a few months

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Backstory:
>Have been dating a girl secretly for a few months (reasons I can't go into).
>One of my friends is one of her best friends and doesn't know about it.
>He had a crush on her for a few years, but she wasn't interested.

Fast forward to a few weeks ago:

>Sitting at my place with girl having a few drinks.
>Said friend and others turn up uninvited (they'd been calling me to come out with them the whole day).
>Friend drunk as fuck, and is obviously surprised but tries to act like he's cool with it.
>He ends up being a fuckhead to her telling her to shut up the whole night, gets handsy with her and slaps her on the ass.
>He slaps me lightly in the face a few times before I warn him not to do it again.
>Girl and I leave to get dinner, come back.
>He is saying lewd shit about continually asking if we have sex etc.
>He slaps her in the face.
>I sit there shocked doing fucking nothing except for asking if she's okay.
>Room mate eventually threatens to choke him out and he calms down.
>We leave.

I regret not punching him in the face every day since that night, and feel pretty ashamed of myself. If it were a stranger I would have, but the fact remains that I shouldn't have let him even touch her in the first place. Even though she said she was okay afterwards, I have a feeling that she just didn't want to make me feel like I did wrong.

At risk of making it seem like a huge drama, I want to tell her that I don't want anyone to disrespect her like that again and sort of apologise for failing. Is this a good idea?

There's also the problem of her best friend (my friend). This guy has driven away at least 3 people from my friendship circle and I've been blind to his negative influence until now. Everyone lets him get away with it and I have been the biggest enabler of his shitty behaviour. How do I fix this, and also try and explain this to my girlfriend without it screwing my life up?

Go easy on me, this shit has been ripping me up inside for the past few weeks.