Pics are good but it's 99% the obvious... Why not just tell him the fix? It's powerdy mildew, treat it plain and simple... baking soda and water....
spray with straight water first, this way the stomata (leaf pores) don't take in the baking soda water too much. Spray them with the soda/water and then rinse them in plain water again. Repeat if needed, that simple !!!
Tablespoon of baking soda to a quart spray bottle is fine... oh and generally you get this from a high humidity... keep it below 70%, I like 60% in veg and 50% in flower... but that's just me IMHO...