I told my friend to delete all of these apps (mistake)

No, removing drivers the old fashioned way wouldn't brick the PC, the GPUs are designed to run in native VGA mode also which require no extra drivers as they are built into the Windows OS. Also perhaps over 15 years ago you would never consider using a HDD/SSD/NVMe from a previous system with new hardware - but these days since Windows 7 onwards it is fine, again basic drivers for chipsets are usually available with the OS so you can easily update the drivers manually.

Not sure why the board suddenly gave problems either.. unless it was a faulty board, was it his old motherboard or bought new? If new he could have got it replaced under warranty - would have been a bit of a wait for it to be repaired or replaced, but it shouldn't cost much that way.

Sounds like Micro Centre did one thing right and reset the CMOS, however sounds like they didn't really fully test it and hooked you with a "needs a new motherboard." Hopefully it is the same CPU and GPU that was in it. I would have personally grabbed a copy of a 3rd party CD or USB windows environment so it can load into Windows via those 2 options and you can diagnose problems this way, even be able to run SFC and DISM which are common tools to spot and repair corrupted system files (I wont mention the names of the 3rd party software as it can be a grey area, but worth remembering just in case something similar happens again).

EDIT: Something came to mind, ask your friend if before the problems started did he try to update the motherboard's BIOS. If he did he may have potentially used the wrong one (although a lot of checks these days will mostly prevent that).