Do you tell your friend her husband cheating on her?

I would probably try to have a talk to the husband about the subject, before I do anything else. At least, it might give him a chance to do something about it. Granted, he might do anything, he might change, or anything like that, but at least the opportunity would be opened to him, whether he takes it or not. If he chooses not to do anything, then I might talk to this friend about it. But it's difficult for me to say, as I've never been in this situation.

I will! and that's the best thing to do in the situation no matter what the consequences would be. though it is hard to do it but i will do it in a way that it would be easy for my friend to understand (how? i don't know yet).

It is easy for me to make a choice whether to tell my friend or not that her husband is cheating on her. I will surely tell her what I saw and it's up to her to believe or not. If it becomes the cause of our relationship to break then let it be as long as I told her the truth. As the bible says, "open rebuke is better than secret love; the kisses of the enemy are the deceitful but the wounds of a friend are faithful." Have a blessed and wonderful day.