Hi Andy
In citadel terms, they are called “shaders”. Nuln oil comes to mind.....
I’ve also heard (and seen) good things about their “Contrast” paint range, which might be worth him looking into.....in fact, there is an almost bewildering range of paint types in the citadel armoury, but the colour range is obviously complimentary to their associated figure ranges.
In my opinion, the Army painter stuff is better suited to historical figures, but again they have a diverse range that can be explored. The washes are very versatile, though similar to the Vallejo I already use. I occasionally use strong tone myself, though as it’s slightly less intense than Vallejo black. I would avoid the “dip” technique though, he’s already beyond that, and as long as it doesn’t become counterproductive and sap his self confidence, the “I could do better” attitude is a good thing as it is what will drive him on to continually improve.
Cheers
Tim