i'd say we mind our own business. unless your friend asked you if you know or suspect something about his/her mate, then you can tell.

would it make a difference if you told or not? yes. if u were proven wrong, then you lose credibility. if you were right, then it's up to your friend how he/she wants to handle it.

either way, if your friend asked you and you told what you know, then there's no need for guilt.