Unfortunately, and somewhat expected, this was not the answer I was looking for. Thanks for trying Praveen196.
I don't need to reset any passwords. He set the account up as a connected Microsoft account. His password is correct and being entered correctly, checking all common mistakes like capslock, etc. ; we confirmed it on another machine. *Just in case, I also tried a user pin that he said he used in some circumstances, which was simply his birthyear... pretty easy and no need to reset.* HP UEFI Diagnostics required a password to "reset Windows" and it wasn't either of the passwords ever used on that computer since he got it 6 months ago.
I could not get into Windows. It would reboot before you could get into an account. No solution anywhere on the interent would allow me to start windows. HP UEFI diagnostic tools said all the hardware was fine. Trying to repair windows reported that windows was not in need of repair. He had not set up a restore point, so I couldn't use that option. He didn't have a rescue disc, but I did download a Windows 10 system disc that I tried to repair it with. I tried removing updates. Everything. Nothing worked.
The only solution was to use HP UEFI Diagnostic Tools to try to "reset Windows". There were 2 options to do this, one that retained user files and one that did a clean install and did not retain user files. I was trying to retain the files, but it kept asking for a password, without indicating which password. I tried both of the correct passwords I had... many times, with no luck. I then tried to do the "reset Windows" without saving user files. This option didn't ask me for a password. It went through a bunch of processes and ended in saying it could not perform the action. No idea or indication as to why not.
Here's how I solved the problem: I removed the system drive (there are 2 drives, the M.2 socket drive is the system drive), bought an external enclosure to put it in, pulled the data off with another computer, and put the drive back in. While I was doing this I noticed a jumper on the motherboard that was attached to pins that had "PSWD" or something similar next to it. Since I was at the end of my ropes wasting so much time on this random Windows 10 issue that appeared out of nowhere, I removed the jumper and restarted.
Here's where I messed up. I should have tried to see if it still asked me for a password if I tried to reset and save user files. I was so fed up at that point I just went straight to a clean install. Whereas before it said it couldn't complet this process, this time it just worked. Everything worked fine, Windows was reinstalled, and I was able to get back into Windows and set up his computer for him.
Windows 10 might be fine when it works, but what an extreme random problem to have... and have no solution for. Windows 10, which I have little to no experience with other than this troubleshooting, also wouldn't let me copy some app data off of his user folder... it was giving me weird access errors and stuff. Maybe a security feature? I wasted a lot more time trying to copy everything because I'm thorough, but in the end just got the following folders that had known files he needed: Documents, Downloads, Desktop.
What a hassle. So glad I use Linux now that it is much more user-friendly to non-computer experts. Windows 7 was the end of the line for me.
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