the ex in-laws were not affected in anyway by Nick's actions. There are other ways about helping someone make better decisions. They seem to have been predatory or vengeful in telling on Nick.

It's even possible that they were hypocrites. If they knew about Nick's actions and only took issue with them after/during the divorce, that would reveal that their "noble tattling" was done with the wrong intentions. Or, if they would've kept silent with all going well in Nick's relationship with their daughter, that too would reveal them to be hypocritical (Disclaimer: I don't know how long they knew about Nick's actions).

Also, I feel VERY confident in saying that Nick did not choose BYU because he knew one day some booster would pay for him to go to Disneyland and get a Jetta. I can next to guarantee he wasn't offered any of this to join BYU. Therefore, I don't look at Nick's ex in-laws as these moral heroes. 

What they did feels as noble as an ex-husband that reveals his wife's darkest secret, entrusted to him during a time when more trust existed in their relationship. In other words, it was all done out of SPITE.