I think it's very disrespectful and inconsiderate of him that he chose to stay with his co-workers all night instead of being with you, even after he promised to stay with you. You're his girlfriend, so you should have a higher priority, but it seems like he prioritized his co-workers and drinking. I also think his co-worker should have taken him home, or she should have called you, instead of taking him to her place; even if he's her boss, that just doesn't seem like the logical thing to do. At least if I were in that situation, I would think that I'm making the situation worse given that this man does have a girlfriend and that it would probably upset her to know that the boss was staying elsewhere.

You should probably talk to him and let him know how all of this is making you feel. Don't let him belittle the situation, he's just making excuses for himself and he's just ignoring your feelings. I think if you just let it go, then he will keep doing it. He needs to realize he has a drinking problem and he needs to get help, and you should give him that ultimatum. You don't deserve to have to deal with this, and I agree that this is a bad environment for your daughter.