I met my friend today and she kept insisting that people who have been cheated on have a higher chance of cheating in their next relationship

I find it unlikely that's true and certainly haven't seen research that would support it.

So, you know, I tend to be flexible about a lot of behavior (casual flirting, window shopping via tinder, etc) that a lot of people are upset by, and his whole, "I'm using Tinder thing for validation" thing is actually something that's really quite common - people use it with no intent to meet.

But he did meet this girl, and it sounds like she thought he was going into it with romantic intentions, which is a problem. And in any event, his behavior is his behavior, and he's responsible for it, and saying, "I did a bad thing because my ex did a bad thing" just means that he's not ready to be in a relationship.