Child, I would definitely tell it. Especially if you respect your best friend and value his/her friendship. If you dont tell, you are not his/her true friend. And the love of your best friend's life is trifling as hell in the first place. He/she cheated with you, so imagine how many others this person may have cheated with. Tell your friend that he/she needs to drop that loser because he or she is cheating. As simple as that.