I don't know... my sister used to work from her bosses house when she would come into town (she normally telecommutes). Sometimes she would stay overnight because it was a huge house. She stopped staying there when he got a girlfriend, though, because she didn't want the girlfriend to feel insecure.
I guess trhe bottom line is: why do you not trust this guy? (I'm NOT saying you should.) Do you have other reasons to believe he may be a two-timing dirty dog?
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?