Yeah, I'm with you on this. This doesn't even need to be accusatory in the first place, just something like "hey, when I was staying at your house I noticed that your housekeeper was sleeping on a mat in the kitchen: just was wondering what's up with that". The guy could decide based on the answer if it was legit ("that's weird; she has a room... it's the third door on the left on the second floor; I'll check on her and make sure she's ok") or suspicious ("oh, she's strange and I think she likes it there") and if it made sense to press more.

I really _really_ hope I'd confront this "friend" if I were in that situation. It's definitely the right thing to do.