Doesn't matter on what relationship is, if someone cheats on me once they are a cheater in my books and there's nothing between us. You can't repair the trust after that. Though, honestly I've never been in such a situation and can't speak from direct experience.
Their past however can be accepted depending on the circumstances. For example I would look differently at cheating on the way out of relationship through a period of separation than a malicious form of cheating to "get back" at the partner.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~