No keyboard or mouse after downgrade from Windows 10

Wasn't sure which subforum to file this under.

My elderly neighbors asked me to fix their laptop. Apparently they were suckered into updating to Windows 10... They said it worked well for 3-4 days but after a few days the screen turned all checkered and messed up and there was a message about "graphics something" (they didn't remember). After that they did a downgrade back to Windows 7 and things were even worse after that.

The machine is an HP dv7 series laptop with an i5 and AMD graphics (Microsoft probably provided a bad graphics driver with Windows 10).

After the downgrade to Windows 7 input devices are not working at all the machine is not frozen, there is activity and it can sleep and resume, but keyboard and mouse do not work. External USB keyboards and mice do not work either (even though Windows shows on the screen that it successfully installed drivers).

Keyboard and mouse do not work in Safe Mode either. Keyboard and mouse does work in Startup Repair though. I ran "sfc /scannow" for offline windows installation and it ran for like half an hour but did not find any integrity problems.

RDP is not enabled so I could not control it over network.

Currently I cannot get past the login screen (can't select user or type password).

There is no PS/2 port, USB only.

It doesn't have a true "BIOS" but UEFI with CMOS Setup-like interface. I did reset that to defaults.

I don't want to do a clean install because I don't want to reinstall their software. I want them out of my hair as soon as possible. (They already called me twice in the past 24 hours, I don't even pick up the phone anymore.) Please help my elderly neighbors.

PS When I Googled I found that this problem is very common but no one posted a working solution.