What stats to raise, what items to equip on each character, what abilities to use and against whom.
Unless he's going to give a lot more details OR post some video of him going through an encounter and showing off what he does right and wrong (and it's probably a lot of the latter) the only sensible advice I can think of is "Watch at least one or two tutorials explaining the foundational mechanics of the game" (like what a proficiency is and how to use spell slots and other abilities).
I think this is the soundest advice. When I got interested in the game for its immersive-sim potential, I made myself familiar with 5e rules and it helped a lot. It also will stop you from blaming poor Shadowheart for missing so often with her Sacred Fire ...
