Is your friend worried about sun exposure to the newly exposed root flare or the small white roots we see around the tree? If you are going to re-cover the root flare with a very light layer of mulch (a good idea), that will solve that problem. If they are worried about the small roots exposed out away from the trunk, those are a very small percentage of the entire root system and are probably of little/no consequence. From what I can tell, I don't see that there was much problem before the work you did (the root flare doesn't appear to have been buried too deeply) and I don't see any problem now (assuming you re-mulch properly).

When you apply the mulch, the layer near the trunk should be very shallow (just thin enough so that you don't see the dirt) and then taper up to a thicker layer (maybe 3" to 4" of loose, fluffy mulch) out at the edge of the mulched area.

i think your friend doesnt know much about trees ...

the active feeder roots.. are probably about as far away from the tree.. as the tree is tall ... they have not been exposed ... so its a non issue ...

really cool lichens on the trunk ...

ken