Not acceptable in a marriage. My gf and I live together, and we both still hang out with friends of the opposite sex; sometimes we take the other one along, sometimes we don't, but she knows the couple girls I hang out with and knows we're not doing anything on the side, and I know the dudes she hangs out with and I trust them too (mostly nerdy dudes who can't umm, do the damn thing like what she has at home

)so we currently don't worry too much about it. But there is NO WAY in hell if we ever get married and have kids that I would put up with her going out and getting hammered with two young children and staying at some dude's house. I'd tell her from now on, to call me and I'll come get her when she's ready to leave the bar and drive her home. And as previously suggested, I would tag along/drop in on these "playdates" and see for myself what's really going on. If she has a problem with any of that, or acts strangely about it, I would seriously get a PI. Your friend needs to get to the bottom of it and get it over with, because the longer it drags out, the more the stress and wondering is going to get to him. Good or bad, he needs to know. Marriage is the most serious, committed level of a relationship you can be in, and I'm sorry, but sleeping at another guy's house, especially with him being single, is not acceptable EVER unless her husband is there with her.