I get that cheating on someone is one of the worst things you could do to your partner, but all these people who go crazy and make their lives revolve around ruining the life of the person who cheated on them typically end up doing stuff that makes them no better than the cheater. If you want to get back at someone who did something wrong, then that's perfectly fine (such as ratting them out for cheating on a test), so long as you don't go overboard or become obsessed with ruining their life.

Cheating on a test and cheating on a partner both involve dishonesty and trying to gain things illegitimately. Running for exercise and running for president are just two ways of using the word running, it's not comparative.

She said it as a joke. She didn't mean it seriously when she asked the question. She just got a serious answer. However, people who would lie and act immorally would do it with either a person or a test, if they're that bad of a person. Some wouldn't cheat on a test but would consider and/or go through with the idea of cheating on a person.

What if they're not in University? OP can turn him in to the teacher and/or principal and they can deal with it. He most likely wouldn't get kicked out, but he would come out with a zero on his exam grade

That is true, they could be in high school or another type of school. However, a large majority of schools have an absolutely zero tolerance rule towards cheating. They will kick you out of that school and sometimes the towns school system, or send you to the town 'bad kid' school.