Mixed RAM: Both DIMMs have the same heat spreader and I "assume" that they are part of the same kit.
Not sure if there were overclocks, but I've already had him reset the BIOS to optimized defaults.
BIOS version is F3. There is a F4 available and a F5 (Beta) available, but not sure if it's worth trying.
Memtest86: I haven't had him try that, but it's a good idea.
Does the A10-7850K follow the distance to TJ Max readings: I don't know the answer to this yet.
Physical damage: I don't know, but I will have him send me high resolution photos and I'll take a look.
motherboard age > sensor failure > incorrect readings trip protections related to those sensors: This is an interesting one. These items never occurred to me before.
What I'm thinking are possible causes, but I obviously have no idea at this point: reset button on case possibly causing the system to reset? I'm considering having him unplug reset cable from motherboard, on the chance that it's causing it. Also, he has windows 10 installed on a apotop s3c 256gb SSD; never heard of this brand before. It was manufactured in 2014. I'm wondering if the SSD is to blame, or the Windows 10 install. The Windows 10 install was upgraded in the past from Windows 7. He bought the computer 2nd hand off a friend 3 years ago and this rebooting problem has been around the entire time.