Regardless, I'm pretty sure it's important to know specifically what this cheating was and whether or not it was actually cheating.
If the cheating is fornication, then obviously the guy is not ready to be married at all. But if it's something like taking another person out because he's not sure about Apple, that's not the same thing.
Well, did both parties have said "plan", or just her; because I've definitely seen situations and been in (both sides of) situations where one party was far more invested in the other and assumed things they thought were implied. Nothing can be taken for granted, you have to ask these things.