What to do when you know your best friend is cheating...

http://hubpages.com/hub/What-to-do-When-you-Know-Your-Best-Friend-is-Cheating

It depends on the friend...

Sometimes, cheating is not a sign of strength, it is a sign of weakness. Someone is not getting what they need out of their relationship, they cannot, for one reason or another get what they need from the other spouse and in a lot of these cases, they give up and go elsewhere. It's a sign of desperation.

They can't or won't leave the relationship for some reason, possibly economic, so it is possible that the person feels trapped.

It is also possible that the other person in the relationship has undergone some change. You have no idea what is going on behind closed doors.

So to shun a "friend" as totally untrustworthy, for having this affair, without completely understanding whether or not it is out of character, and not placing yourself in their shoes, is a rush to judgment.

It is quite possible that this friend is in pain and instead needs a little support.