Friendship advise?

Me and my ex boyfriend are best friends. We have been broken up for a couple years now. I am now in a new relationship and so is he. He broke up with me and hurt me really bad but we got over it. While we were dating I cheated on him. He is now hanging out with the guy I cheated on him with so I was wondering if I should tell him before the other guy does. I know it will hurt him but I would rather him hear it from me then from the other guy. Should I tell him or keep it to myself?
Friendship advise?
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