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.