This is why updating a BIOS isn't always "safe" — things can and do go wrong at times, even for the most experienced among us. I had a Gigabyte (or maybe it was MSI?) board back in the Ryzen 7 2700X days that killed the 2700X when I updated the BIOS. The flash appeared to go fine as well, and swapping in a different AM2 chip showed that everything else was working. I ended up having to replace the CPU.
The biggest failure points, in any build, are usually (in order of potential):
- Motherboard
- RAM
- PSU
- CPU
- GPU
- SSD
That's assuming the problem isn't just "user error," which is actually the most common problem in my experience. But the fact that this builder tried replacing the CPU and GPU before trying different RAM is interesting. If I had to guess, it would be that the BIOS update ended up with an unstable RAM voltage and possibly fried one or more of the sticks — assuming it's the RAM, which at this point seems like the only real possibility remaining.