I think you have this backwards. The waiting period you are talking about is AFTER returning to US, not waiting outside US. If he returns to US in TN status, he should wait 60-90 days before applying for GC, and then not leave US at all until hegets AP and EAD (he should be getting familiar with these things, it's his issue).
The important thing is to not LEAVE US after marrying, so whatever this thing in Mexico is, if it does not make him married. GOOD. He neds to marry in US and stay put until he processed his AOS.
The so-called waiting period really doesn;t apply to TNers marrying US citizens, it is more for TNers getting GC through employment, so I would not worry about this.
But there is absolutely no reason he should remain outside US to "prove" anything. He needs to be in US.