A friend of mine loaned a gun to his brother because the brother said he wanted to learn to shoot. The brother went home, laid down on his bed and shot himself in the head. None of it was my friend's fault, brother wanted to die and would've done it without the gun. But my friend still lives with "what if I hadn't loaned him the gun?". The point is, you never know what the person you loan a gun to has in mind. But I would loan a gun to my father and my wife. No one else.
Sad as your story is, your friend's mistake was not loaning his brother a gun, but loaning it to a person with the premise of "learning to shoot" and letting them take it home unsupervised. The brother, unfamiliar with guns, could have just as easily shot himself in the head by accident. Your friend would still have the same guilt, even tho the brother did not have the same intent. I doubt if many of us would lend our car/truck to someone under the premise of "learning to drive" and let them drive off by themselves.