Another smelly Audi transaction. Years back I purchased an Audi as a private buyer from a private seller but who also owned a so named considered '
reputable' high-line import repair shop. Was told it was his wife's car and where he took exceptional care of it, blah, blah. I'll skip the details, but my fresh purchase turned for the worse. Trans started to slip within a few days to a complete kaput. I PAID to have the trans rebuilt (elsewhere), quickly learned the odo was spun and with clever detective work I was able to score the never to release / disclose dealer service records (why is that dealers?)and it was a slam dunk case against the scumbag. If it wasn't for the discovery, I would have been S.O.L. This '
mister reputable' played the wife has cancer card, postponed and delayed court action. It only made it worse for him, dragged out awhile but he lost. I could have pushed and the law would have closed his biz and him possibly risking jail time. By the way, I did have a bill of sale where I signed as buyer 'as is and as shown'.
With that, the OP BIL needs to have a clear conscience and make sure he's properly disclosed any odometer issues... if any. Other than that, he's probably safe to say that seller is now the proud owner of a leaky Audi

. If it was a dealer sale, thats another matter and may have specific time (ex. 72 hours) to null the deal.
Lastly, a final tactic for a buyer to get out of the transaction is to claim
'under the influence' during that time. Difficult to prove, but cases in that scenerio have favored the buyer. So don't share a beer with the buyer during the transaction.