What exam is he sitting for?
Norm Christopherson's book, HVAC Technician Certification Guide, featured at the top of this web site page, is a good practice manual for taking tests such as the NATE and the RSES CM exams. Also probably wouldn't hurt to use it as part of preparing for a state license exam, but for that I'm sure he's gotta know more than just HVAC particulars, such as building codes, business practices, etc.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.