I would like to know what you think this fellow is taking responsibility for, and why. If you are saying that he was responsible that the firearm was stolen, then wouldn't he be responsible if a hundred dollar bill in the same location was stolen? It really seems here that you are blaming the victim.
Now you might contend that he wasn't prudent, or that he was foolish, but I don't think you can push responsibility for the act on him. Why would this be any different if he had left his car unlocked and the kids stole that? Would their stealing of the car be his responsibility? I contend that in a civilized society, you should never be responsible for the actions of others, unless you deliberately and willfullly used fraud against them to get them to do what you wanted. The only exception would be if you have guardianship of a person who is not responsible for themselves.
EBUCK