Gonna give you my 5 cents:
If he wants to learn to sculpt skulls he should get reference of actual skulls. The reference they have there is fucking horrible.
In terms of sculpting techniques they should start with a hard brush like the clay buildup brush in ZBrush. Build the forms with that and DON'T SMOOTH EVERYTHING. Just the same way you should work when you're painting. Instead you go over your hard, chunky strokes with a smaller hard brush to get some nice variety into those strokes. I'll give you a video on this:https://www.youtube.com/watch?v=0PaYUUvgwYM
They should also learn that the eyes are positioned about halfway on the face. Not 20% from the top. Your friend should also break symmetry at some point. You wouldn't post a painting where you've just copied the left side to the right, would you?
What I'd also like to see is something they've taken A LOT further. I'll show you my one of my first sculpts I did in Mudbox in 2016. I'm posting this because I made a lot of the same mistakes as OP:s friend but at least I tried to make it into something more interesting with a bit of Dreamworks expression and detail.
One more thing: If you really want to get real you should get ZBrush. If you want to get good at sculpting you're going to have to learn it at some point any way so why not start now?
Hope this helped.