Ask her to imagine the following: let’s say she ends up being really close to your spouse one day, like she couldn’t have imagined a better pick. The two of them become super close, but as the years go on, for whatever reason, you end up having an affair. Now ask her if she would go no contact with you because maintaining a relationship would condone your actions and ignore your spouse’s suffering.

If she says no, then maybe she could see a bit of where you’re coming from. If she says yes, well then I would wonder if a lack of empathy was part of the reason your dad sought a relationship elsewhere (not that that justifies cheating).