I've worked for some of the biggest professional service firms (so not law, but similar environment) and you'd be surprised re the sharing thing.
I've had to attend mandatory training where I've had to share a flat with a colleague. I did once share with a man but that was because we both got on and neither of us wanted to share with a complete random and - perhaps crucially for some of you here - he spoke to his wife first to make sure she was onboard, and he would have sucked it up and shared with a random bloke if she hadn't been OK with that.
I've had to attend mandatory training where some people have to share a room. The firm booked X number of rooms for X+many people and basically left it to the staff to sort it out. Luckily, there were enough blokes who were happy to share together that it wasn't an issue, but I was prepared to quit over it. A serviced apartment is one thing, but sharing an actual room with a colleague is pushing my boundaries to a point that I'm not happy with.
The drinking culture is big.
I can see a situation where a bunch of people got completely wasted and couldn't make their own way back to their own room and ended up crashing in someone else's. That part is completely plausible.
If there were two men and two women, and there was a (secret) couple in the mix, I can see why the couple would have refused to share with someone else. So, I see a scenario where your husband only had a bed availble to him that was occupied by a random women. But...
...THE FLOOR WAS ALWAYS AN OPTION IF HE WAS TOO HAMMERED TO LEAVE. In fact, if they were that hammered, sharing a bed was a bad idea, as one of them could have easily puked on the other.
When you're drunk, you can end up in bad situations, but anyone would then attempt to make the best of the a bad situation. In this case, it would have been giving the female colleague the bed, and just sleeping on the floor, i.e. not sharing a bed with another woman.
I wouldn't share a bed with any of my male friends (who are like brothers!) let alone with a random man. If I really didn't have any other option (and I feel like that isn't true, because surely there would have been a receptionist available to look up my room booking and get me into my room?), I'd crash on the floor. I guess I wouldn't get myself drunk to the point where I couldn't safely walk back to the reception and ask for assistance.
If there was no receptionist, we're not talking a hotel with rooms, we're talking flats. In which case, there's more than a bedroom, there's at least another room or a private corridor, and he could have crashed there.
At worst, he's cheated. At best, he's an idiot. Neither truth is particularly appealing to me. Unless he's in his 20s in his first professional job out of uni and hasn't yet worked out he's a grown up, I don't think I'd be happy with drinking to the point where he ended up in this situation. Even then, I think I'd start to get the ick.