If you had no indication of it, had no reason to suspect it, and the info came from someone known to lie, had a known agenda to cause trouble and had already tried to cause serious trouble for other family members, by lying.
A family member by marriage living in another country has just done this. Her agenda is largely around causing significant problems for her ex-DH who is related to DH and an attempt to stop a family trip from happening. She had already tried other things to stop it happening but was unsuccessful. She has chucked this in at the last minute, knowing if it were believe, it would ruin the trip. It allegedly happened during a trip abroad that DH went on with her, her exh (his brother) and she claims the affair happened with a friend of hers, 18 months ago.
My first reaction was that she was lying to cause trouble. DH denied it. He assured me he did not, would not. It is his word against hers as it happened at a time when I was not with him for two weeks and my experience suggests that as he is my DH and is not known to lie and i trust him, vs her already established reputation as being a someone not to be crossed if she is hurt (even prior to their very unamicable split) I am inclined to believe him. And of course, I absolutely want to believe him and I have no reason to doubt him, he has never been unfaithful in our relationship before, has been away from me before and it has never crossed my mind he would be.
What would you do? Leave it? Try to find out more info? She has not given me info, but said there are photos and I should ask her friend. My BIL said she is just trying to cause trouble, but of course he is going to say that! But, as I said, she has a very recent history of causing trouble and being very nasty due to their bad split, and before that, so it's not like she is someone of good behaviour who is usually lovely.
I would like to let it go. Would you? Could you? Should I?