let or her see logic

If the guilty spouse is still inlove with you and did not have a relatioship going, meaning it was a one night stand.
Ask the guilty one if he or she could really be trusted by the person that they slept with, or if they think that,the person in turn could trust them, were they to leave you for that said person. Cheating is self defeating, it shows us and whoever we cheat with that we lack loyalty.