I would not tell their partner unless I knew they were engaging in behavior that would likely result in an STD being transmitted. With that being said, I wouldn't be friends with anyone who cheats. If they want to cheat, that is their business, but i'd rather not be in the awkward position of knowing about it! If my friend had that little respect for the person she/he was with and "cared about" I would be concerned for their character. I would fully expect this person to screw me over eventually. Friendship off.
There are exceptions, but generally this is how I would react.
A real friend wouldn't have to ask that question.
talk to your friend about they r doing not their spouse.u'll lose ur friend.people cheat for different reasons all the time. if someones cheating theres something missing in the relationship
it is individual choice either to have relationship or not, unless you know complete background for the relationship I feel it should not be informed to their partner
No, but you should tell her to work it out. If she doesnt tell him, its not your business.
Only if you want to lose them both as friends.