Past criminal behavior is routinely admitted as evidence in court.

It's not proof, but it is evidence.

You're refusing to see this, by the way. You're capable of understanding what I said without me saying it.

It certainly doesn't help his case. If you have had troubles with the law prior to a new crime, it gets taken into consideration negatively by the judge. Same with cheaters. It means he is a cheater by definition and has the moral compass to cheat again.