Judging by the product photos on the Stack-On site, it's pretty similar to my own unit, although mine came already assembled.
Although Stack-On uses hard-to-duplicate keys, breaking into one of their sheet-steel cabinets should be pretty easy. So if his is closed, and he needs it open, a strong pry-bar will do the job. Or, he could just disassemble it from the outside.
If it's only the lock that's stuck, but the case is still open, Stack-On ought to replace the lock at no, or low, charge. He might have to send the door assembly back, though. I don't believe that the lock, itself, can be easily removed—but I could be wrong. Maybe a local locksmith can replace the lock economically, if your friend brings him the entire door. A "house call" would be expensive, though.

Those locks are easy to replace. Just one big nut on the inside (assuming you can get the door open).

Sometimes the springs will stick and one of the pins will not go to the open position. Try spraying some WD-40 (or other spray lube) in the lock and work the key in and out a few times. You can also try working each pin in the key opening with a small object (like a T-pin).

Thanks for the advice gentlemen. I'll have him give it a try.