What is he running for ignition?  MSD?  What coil?  Trying to think of things that can change suddenly.  Did he do any other work right around when the timing issue started?  If the TDC check confirmed the damper timing marks and the distributor rotor, then it doesn't sound like a distributor gear.

Does he have a boost retard system as part of the ignition?  I don't think I would drive it hard or possibly at all until it is sorted out to avoid melting something.

What is wrong with running it at 16 like he did before?  Maybe he blew a power valve and is trying to overcome the rich mixture with timing.  I don't know how a Paxton type system needs to be set up.  What compression is he running?  I would be afraid of melting some pistons with that much advance.  What fuel does he run?  When the timing is checked you are comparing number 1 firing with respect to the crank.  Any problems with the timing chain, etc., would show up when looking at TDC, I think.

I assume if it is a points ignition without Pertronix that he has checked point gaps to ensure that dwell is correct?  One of the reasons I asked about MSD is that some timing lights that allow you to dial back the advance don't handle multiple sparks - they will work OK at zero degrees but will give false timing when dialing back the advance to TDC.

I find that a sudden change in a machine's behavior often follows and event or some work that was done that was seemingly unrelated at the time.  When you hear yourself saying "it can't be that" move it up higher on the troubleshooting list and prove that it can't be that.

I vote for a large vacuum leak somewhere.