Should you tell him she's cheating?

Maybe she was having a meeting or just met an old friend or something. But if they were doing something that makes your doubt for sure true then yes you should tell her husband.

I might speak to her about what you saw and ask her what's going on. I might say "Hey Sue I saw you at the mall with that handsome guy and I'm feeling uncomfortable about it. You know I like and respect your husband and like you as a couple. I hope everything is ok between you two." Then the ball is in her court.It might be innocent.Or it might be that she is cheating. Once she knows that you know she may reevaluate what she is doing. I do not think it would be ok to tell the husband. For one thing if he is your friend now he won't be any longer and if you are wrong about what's up you may cause trouble in their marriage.And she will not linger be your friend either. I say if someone is in danger than you butt in.if not stay out of it!It's really not your business.Please don't be offended by that statement .....I don't mean it that way. Raydene

I think it depends on how close with her husband that you are. How bad would you feel whenever he found out, knowing that you didn't tell him? Personally, I wouldn't get too involved, but I would tell him that he should check on who she's going out with more. You don't want to jump to conclusions because that guy could be a relative.