It sounds like the best thing your friend can do is sit her husband down and insist on an honest conversation. Tell him she's noticed no money has gone out for hotels and ask him to explain. I wouldn't advise her to ask in anger (or at least not to show it if she is angry), it gives him more excuse to become defensive. The two of them need to discuss this whole stuation, she needs to tell him about her concerns and he has to come up with some answers. No amount of speculation by you and her, or by people on ab, about whether or not he's having an affair is going to help this siuation.
I hope your friend gets it sorted, and either way she'll be grateful for your support.