I am in a similar boat to your friend. 8 years out of the cockpit and looking to head to a regional next spring. Here is what I am using.
These are not original thoughts, there are ways that military pilots in a similar situation to myself to be successful. YMMV
Everything Explained for the Professional Pilot - Richie Lengle (There is an iPad version also, that is very good and just updated)
Pilot's Handbook of Aeronautical Knowledge or another book on Aero.
Instrument Approach Procedures Handbook
I would also suggest studying some Jeppesen Approach Plate tutorials online, or hiring a local instructor to give him a hour or two "ground school" on current Jep plates and how to brief them. I also purchased Ready Set Takeoff Preview, but that is more in hopes of getting on with a major soon after getting current. (I know, I know. You never know how long that might take....) Good luck.