Under the circumstances, Swampwitch does have a valid point. You can give him the option to tell her about it first. you don't have any proof against him other then your word vs. his.

Problem is, does your other half know if he was cheating on her before they got married? Cause that does count as proof.

But if you don't tell her, that is stress and dishonesty to yourself that could ruin the friendship. Everytime you 4 are together, you'll think on what happened and eventually, you'll end up looking for reasons to not see them.