If a woman accuses her husband of cheating, and he isn't, do you think maybe she is?

I have a friend who this happened to. His wife now says she won't sleep with him because she is "sure" he's been unfaithful. I am fairly certain she's wrong, and I think she's saying it to cover her own tracks. She travels on business constantly, usually with the same male coworker, with whom she's thinking of going into business. She's been very COLD towards her husband for years.