I think there are many valid reasons to contact someone from your past. But I think it should be avoided in situations where that person mistreated you as in your case. This is the type of situation where you learn to forgive. Everyone goes through something like this but on different levels of severity. There is no need to tell that person you’ve forgiven them because they likely won’t understand and appreciate it. Believe me, in time it’ll catch up to this guy. Cheaters don’t just become loyal. He’ll probably cheat again and again and at some point it’ll start hurting. And maybe then he’ll reach out and ask for forgiveness. That’s when he’ll understand and appreciate it. Until then just forgive and find peace within yourself and let karma do the rest.