In my experiance these programs are not long enough to really get enough information about one one system let alone dozens. I was the GM at a diesel fuel shop for 4 years and before that turned wrenches from before getting out of highschool.
I would offer this advise... if you don't see yourself doing work in the field for the rest of your life, you dont think you can take standing for 10+ hours a day, the pains, the aches in your hands from using tools, RUN AWAY!!
The field seems to be something you can very easly become trapped in. Being a mechanic or diesel tech does not relate to other jobs that are not very similar. I feel that I was lucky to get out of the field, because if I didn't get the chance I would have been doing it for the rest of my life.
If he wants a job in the field I would encourage him to stay away from the heavy on-road truck field. Its truly a PITA!! If I could do it all over again I would have tried to get an internship with Caterpillar or Komatsu or even Liebherr. Working on equipment in the off-road mining industry is a pretty good field to be in. Up-time for the industry is key and the mechanics/techs are paid pretty well.
The nice thing is if you can work on this off-road equipment you can work on the on-road.
As someone brought up above, these schools are good if you want to get on with a manufacturer. Also check out Mercedes, the dont want you to know jack about working on cars... they prefer to train you to work on their cars.
Just a little background about me, I am 29yo and started getting paid to work on cars/equipment when I was 15. I hold ADS, ASE, Bosch, Stanadyne, Refridgeration, and other certifications. If you have questions post them, I am happy to share how things worked out for me.