There are lots of things going on that would make me un-comfortable. The worst of which is that a 45 year old man goes to a party without his current girlfriend (K) and gets drunk enough to not be able to drive. Since he called K to tell her about this, K should have insisted on coming to get him right then and there.
The second part is that although K is his girlfriend, H has not yet introduced her to the ex or the exe’s brother (is that correct?). There is a teeny tiny possibility that the ex is OK and would not have any intentions of getting together with H and she hopefully would insist that H sleep on the couch or in a guest room, but I sort of doubt that that is the case. One of the 2 people in the original relationship probably still has feelings for the other one. Just not sure which one it is. Sounds like a case of the first relationship not being completely over yet.
H knew he was going to a party where there was likely to be drinking going on (unless he’s naive or stupid) so he should have made a plan for a designated driver to take him home, beforehand. Sounds like H is using the drunkeness for an excuse to stay over at his exe’s house.
H may have an attachment to his exe’s child (but probably not a big of an attachment as he is leading K to believe). He’s probably using the kid and the drunkeness as excuses for why he needs to be over there.
H may consider his exe’s brother to be his best friend, which is fine, but because H and his ex are broken up, they should have decided it was best if H only visited with his exe’s brother in other places. He could even visit the child in the presence of his exe’s brother, considering that that person is the child’s uncle. There is simply no need for H to be over at the exe’s house. If H and the ex had been married and the child was theirs, together, I might have a slightly different answer, but even in that case, H would have to make it quite clear to K that if he spent the night over there, it would be on the couch or in the guest room and he should make it a point to introduce the ex to his current girlfriend so everything is out in the open.
The worst part for you, is that K is not likely to listen to your concerns. I know this by experience, that girlfriends only want to hear good news about their naughty boyfriends. They don’t want to hear negativity and they are likely to drop you as a friend if you point out the likely scenarios (even if you have direct proof that there was monkey business going on).
All you can do is gently suggest that she should have a nice talk with H and maybe ask him to introduce her to his ex and suggest that if he ever goes somewhere and ends up drunk that she would prefer to pick him up rather than having him spend the night. She’ll probably get mad at you and say something like, “I don’t expect you to understand! H is not like that and I trust him.” That verbal slap will sting, so be forewarned.