Troubleshooting for a friend

Hi All - been awhile since I've been here. Hoping you can help me help a friend. He built a computer on quarantine and has been having some issues. His specs are as follows:

ASRock B450M Steel Legend motherboard
AMD Ryzen 5 2600
4x4gb [16 total] Viper DDR4 2666hz ram
RX 5700 GPU

His main issue currently is that windows and his BIOS recognize that there are 16GB of ram; however, Windows says only 7.9GB are usable. He is running 64 bit windows 10. He has done the windows memory tests and everything has come back fine. Does anyone have any ideas why only half of his RAM is usable?

We did troubleshoot and verified that all 4 sticks can run 4x2 for a total of 8GB.

A few years back I had a similar issue and it turned out that my CPU was not running in dual channel or not all of the cores were engaged (sorry for my lack of technical expertise, going based off what I vaguely remember microcenter telling me the issue was). When I swapped out my CPU, then I had all 16GB (admittedly I can't remember if it said I had 16GB anywhere or if it just said 8GB).

I am wondering if there is a way to trouble shoot the CPU or am open to any suggestions at this point as to what else this could be.

Thanks for any help in advance.

Edit: It says his usable RAM is 7.9GB and "hardware reserved memory" is 8.1GB. I have no idea what that means.

Second Edit: he did what a lot of the youtube videos out there said to do in terms of unlocking the hardware reserved memory and that did not work either.
nice pretty helpful
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