Iwonder08
I'm actually with you. I would not tell the girlfriend, I wouldn't lie if she asked me and I wouldn't have her round for dinner with this nonsense going on, but what is needed here is a far greater degree of separation and respect between mother and son, and the rest is up to him.
I would, like others have said, set boundaries at home. I'd take my key back, I'd say I never want him around again in the evening or staying over or bring mates over, and to call first if he wants to come over. I'd tell him he'd put me and my dd at risk with drunk, drugged up (inevitably) strange men. That's utterly unacceptable and would never allow that to happen again. He needs a very blunt message on that front.
Other than that, I would back right off. Who knows if he is immature, has some type of sociopathic/non-empathic tendencies, or just behaving like a 'lad' in our culture. I spent a lot of time in my twenties with a large group of male friends, and I think, judging from the replies on here, that people would be shocked at what was fairly standard behaviour amongst nice middle-class would be engineers, doctors, they weren't all City boys at all, and they were all the type of men who seemed faithful, nice, caring, wouldn't be the type to be Jack the Lad. Over the years, most of them at the very least snogged someone else or had a lap dance/something worse on a stag do. Some were fairly unfaithful, some just overlapped romances in a serial monogamy way but hurting people along the way. Almost none were totally faithful as in really did nothing in their twenties and most had girlfriends.
So- I'd leave him to it. I wouldn't call her, I wouldn't interfere. I'd pray that he starts to realise that if he won't do things like be faithful for moral reasons, he'll do it because the consequences are bad- you don't want him staying, his family don't like him, he'll lose the love of his life once for a silly mistake. Perhaps he uses a lot of coke and he might get treatment.
You can't guilt him into change, and you aren't his therapist. Have far stronger boundaries around protecting yourself, your own life and your other children from his stupidity, then stand back.