25,000 feet is very high for a mooney. Sounds like he was trying to fly as high as possible, then passed out, the plane kept climing then entered a spiral dive.
I'm sure a lot of pilots go above 10,000 feet, or even 14,000 without oxygen. It's a tricky subject. It's not like as soon as you go about 14,000 you will pass out, but you do need to keep track of how long you've been at such a high altitude.
I may think twice the next time i'm tempted to try and find the absolute ceiling for an aircraft.
25k is a normal ceiling for a decent turbocharged IFR tourer.
I fly a TB20 which is non-turbo and has a 20k ceiling and I've been up there a few times, and routinely go to 18k or 19k as necessary to remain VMC above weather. Recently I had to sit for 6 hours at 18-19k, to stay above weather.
This pilot would definitely be on oxygen.
His oxygen may have run out (you will need a LOT of it at 25k, using a mask; he would have had a fitted o2 system but regular flights at 25k will deplete the cylinder pretty quick, and refills are not available everywhere, even in the USA), his 1st stage regulator or demand regulator may have failed (there is a well known brand of electronic demand reg which fails SHUT when the battery goes flat, although it does give you lots of audio warnings), or of course he may have become otherwise incapacitated.
At 20k, one merely needs to stop breathing for say 20 secs it takes to do a long ATC readback and one feels the need for another good long sniff. At 25k, any oxygen issue and you will be out of it pretty fast.
I don't think one could get a non-turbo plane (with typ. a 20k ceiling) to go to 25k. The engine would not be making anywhere near enough power to stay above Vs. I reckon my TB20 would make 21-22k absolute max, at the lowest possible weight. The plane flies just fine (the turbocharged TB21 will make 25k) but engine just runs out of steam, and you stall. I've tested this a few times. If the pilot was unconscious when this happens, one would be in a spiral dive more or less instantly because no plane will recover from a stall by itself with wings level and keeping them level, and no normal plane will recover to wings-level from a major roll axis disturbance in the absence of a pilot. Also the autopilot (if used) would have wound the pitch trim all the way back, to maintain altitude (or VS, or whatever was the target) so when the full stall actually occurs, the nose isn't going to conveniently pitch down to let the speed build up (Turkish 737 at Amsterdam e.g.) because the plane has already been trimmed
for Vs.
IMHO, pilots who fly IFR (airways) without oxygen are less than smart. At typical European airway MEAs (FL100 is where the routings start to work OK) not only are they not functioning optimally (esp. during a long flight) and arrive knackered, but they are also cutting off a huge portion of their operating ceiling, which is by far their best weather handling strategy. A portable o2 kit cost peanuts, relatively.
Oxygen is desirable at 10k for long periods, 13k for almost any time, 15k absolutely, and at 18k few pilots would be functioning. At 25k nobody would be functioning (not saying they would pass out totally) after some minutes and looking at the average GA pilot fitness probably a lot sooner.
ot so ! climbers have climbed mountains higher than that without oxygen and with exertion which pilots dont have.
Mountain climbers go up slowly, over days, and all the serious ones are super fit. Even climbing that little hill called Kilimanjaro (19.7k) which is just a straight walk up a path, people just manage it over 3-4 days on the way up, with a few hard core types doing it over 1 day. But a turbo Mooney can get to 25k in half an hour.