I agree with those who said "It depends."
If there is still pain over an ex... even if it ended with honesty... I don't think friendship is wise.
If there was cheating or dishonesty... I actually think forgiveness can happen and with forgiveness can come friendship. I forgave my ex who lied and cheated because they made an amends that was genuine. I know it is wise to forgive regardless, but the amends makes it easier to move to friendship. I am not ready for a friendship with this person because the situation still causes me pain. But one day... when there is no pain... we may be able to be friends. Only time will tell.
I think the hardest for me has been to remain friends when someone leaves your life without a goodbye. Even if you hurt or upset them... going without a goodbye just seems so mean. But again, if they no longer cause pain... then friendship is possible even in this situation. Although, it would be hard for me to trust that they wouldn't just decide to leave again with no notice... and I guess that is a form of pain... so this one might not work for me.
So nothing new here...
it depends.