
According to Michael Musto and several folks I actually know, one should never tell a friend that their boyfriend, girlfriend, wife or husband is cheating on them. Michael believes at the end of the day, it is the messenger that gets shot (click on his name for the full post).
Catching your friend's significant other in some slide-off action really sucks. Unless you know for a fact they are in an open-relationship or not really serious. Telling them about an incident of infidelity is truly horrendous, because they will be devastated and embarrassed. Yet, I don't think helping with the lie is something a friend should do.
I think it is all about how you deliver the message and what sort of support you offer after delivering the bad news. The way I see it you would risk losing the friendship or feel like shit when he or she finds out you knew about it and said nothing.
Good people, share your thoughts with how you would handle this sorta situation.