If I have proof my husband has been cheating should I tell the woman's husband?
If you have absolute proof that your husband is cheating with this other woman then you should deal with your own problems with your husband first and yes, the other woman's husband should know. This is not an uncommon problem and most people (whether a husband cheats with another man's wife or a wife cheats with one of her friend's husbands) is very common and the victim of the cheating generally does tell the husband or wife that their spouse is cheating with their spouse.