Though this is geared towards OC'ing and a 5950X, this selection should be more than acceptable for a 5900X.
View: https://www.youtube.com/watch?v=whuXXSZ6pJs
MSI MEG B550 Unify-X
Gigabyte X570 Auros Master
ASRock X570 Taichi
ASRock B550 Taichi
Gigabyte B550 Auros Pro V2
B550s are apparently better OC'ers than X570, so that's why there's more 550s there. If they're not looking to do that, then it doesn't matter.
Oh, and the lack of a chipset fan on B550 may be a plus.
Mobos are about their feature sets - it's not limited to VRM cooling:
-Does it have enough SATA/USB ports?
-Need WiFi/bluetooth?
-Need dual bios?
-Does it matter what the memory topology(daisy chain/T-topology) is? [For extreme memory OC]
-Does it have enough PCI slots for add-on devices?
-Does it have a Q-code display?
-How many fan headers does it have?
-Does it have RGB, or ARGB only headers? Does it sport both?
^Things like that, for example.
If your friend doesn't know what he wants out of one... well, it's real easy to over/under spend - usually the 'over' happens, and there's things he'll likely never touch during the board's lifetime.
Trying to simplify what's the most complex part doesn't work too well. Probably best to find reviews of boards - or a list like in the above video - that have more of what he needs and less of what he doesn't.