What are the ashrae books that your friend brought to exam? I found 2009 FUNDAMENTALS and 2010 REFRIGERATION so far...would that be enough?
He brought all 4 books (Fundamentals, HVAC Systems & Equipment, HVAC Applications, Refrigeration) to the exam. I don't know how much he used each one, but I know the Fundamentals book was indispensable.
The years aren't that important for the Handbooks, just try to get a set of 4 consecutive years (i.e. Each book is revised in a four-year cycle, starting with the Fundamentals. The most recent cycle is 2013 Fundamentals to 2016 HVAC Systems & Equipment). I can't speak to the importance of the other 3, but the Fundamentals is critical.
As for courses, I took the PPI Mechanical Online course with Richard Davis [I don't he is teaching anymore
]. IMHO, it was the reason I passed. It forced me to study, and Mr. Davis was really good at explaining subjects that I had not seen since college. I would highly recommend a study course from PPI for the Mechanical exam (I have also heard good things about Dr. Tom's on this board). As always, YMMV.