It's your lucky day because I just went through this process. Here's a lightly edited version of the message from HR is below. Also, technically his active duty time in the guard should count. Practically speaking, I haven't figured out a way to make this happen because you don't get a DD 214 for active duty for training time.
"You may begin the military buy-back process by first obtaining your Military Earnings Statement. To do that, you will need to complete and submit the attached “Estimated Earnings During Military Service” form (RI20-97) to your military DFAS Center. Please note that a copy of your DD-214(s) is required when submitting the RI20-97 to DFAS.
For your convenience, the following web link provides a listing of the DFAS Centers for each military branch;
http://www.dfas.mil/civilianemployees/customerservice.html#Military. Please submit the RI20-97 along with a copy of your DD-214(s) to the appropriate DFAS branch for which you served."