Susan says

AI don’t think it’s fair for your best friend’s husband to ask you to keep this secret. On the other hand, if you do tell his wife, she might be tempted to turn her anger on you because you’re the one delivering bad news. Plus, if the two of them stay together, he’ll never forgive you either. My advice is to give him a deadline to tell his wife – making it clear that if he doesn’t tell her, you will. That way, the problem stays between the two of them, and it’s up to them to sort it out without involving anyone else.