I've seen varying quality in t-belts, especially comparing oem vs replacement. due to CA laws, all cars are required to reach 105,000 before a Tbelt must be changed. don't recall when this started, but it was after '97.
I changed my 97 subaru T-belt at 60,000 in 1999, and it looked brand new with minimal stretch. the name brand replacement made it 30k before it stretched to the point where I was getting misfire codes and rough idle. It was a gates or a GY. But I didn't know what it was. at 106,000 the WP dumped so I pulled it all apart. Bought an oem belt. It was significantly sturdier than the aftermarket, which had stretched a good bit. All the misfire codes went away, and the healthy idle was back. As such, now I am firmly an oem t-belt person.
Belt condition is impacted by age, mileage, and how hard the engine is driven. If they get into the pedal, more force is required to pop the exh valve open. Heat, oil seapage, dirt... all impact wear. I once looked at a volvo with oem tbelt at 140,000-- It looked like a dead snake rotting in the sun. He swore it had been changed until I popped open the cover ("I didn't know there was a belt in there"). walked away, it could have gone at any moment.
A t-belt job runs $350-700 in general. Crashed valves/head $1200. Shattered pistons... and you're looking at $2k+. Give your guy those numbers and then let him roll the dice.
And perhaps stop changing his oil. That engine is gonna go when that belt does, and you don't want to be associated with it. I'll gladly help out family, but if they aren't the type to listen to what you suggest, after relying on your expertise to keep things going, I'd be wary they'd blame you if/when it pops.
Or at least get them to sign a letter acknowledging you've told them this and they are aware of your recommendation. That may be paranoid, but I've seen family do really nasty things when $ and blame are involved.
M