Most Helpful Guy
It's not that weird but it does depend on the situation. If my girlfriend had a long term friend a long way away and went to stay with him then that would be fine. But if she had a friend in town and wanted to spend the night I'd say it was a bit weird... don't people stop having random sleep overs at about the age of 16?
Really the question is, how much do you trust him? If my girlfriend stayed over at a (male) friend's house then I'd think it was weird but I would still 100% trust her that it was purely a normal friendship thing. However the fact that you have asked this question suggests that you are a little suspicious...?
To put your mind at ease a little though... him sleeping over really doesn't indicate at all that he's cheating on you. Let's look at this realistically. If he wanted to have sex with his female friend then he could do it any time. Why would he choose to do it when her parents are asleep in the room next door when he could just meet her at an empty house in the day, or do it outside anytime, or whatever. So no, I don't think they are at it like rabbits if that's what you were asking, but I do find it slightly weird lol.