But if the unbelieving depart, let him depart. A brother or a sister is not under bondage in such cases: but God hath called us to peace.
-I Corinthians 7:15, KJV
Let the cheater leave!
If a cheater is hell-bent on denying Christ by their actions, let them. This is on them. I see this verse giving us, faithful spouses, the freedom to move on with our lives in such situations.
Trying to control someone hell-bent on adultery is a recipe for misery as we do not control someone else.
It hurts. Sure.