I kind of understand his feelings. It's really disgusting to get knowing that your (even at that time) ex-GF has slept with your friend.Especially with your friend, not just with some stranger. It hurts, no matter what. May be in your mind you know it's not a big deal, since you weren't together, but what to do with your heart? Sometimes we can forgive but not forget...
i think the guy just needs some time to deal with that. Only time will show whether he is able to accept it or not. And his decision will have nothing to do with all our opinions.

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"What lies behind us and what lies before us are small compared to what lies within us."
Oliver Wendell Holmes