It's hard to imagine any way that the father would not be found at least partially responsible.

But ~ I would be disinclined to punish him with anything more than a suspended sentence. He's undoubtedly punishing himself, and will continue to do so for the rest of his life. He does not pose a danger to anyone else in the future.

Now, for the idiot(s) who provided the venue where Dad was able to exercise such poor judgement ~ and who undoubtedly encouraged and reassured both father and son ~ no legal penalty available in any modern society is harsh enough. They need to be stopped before they kill again; a slow painful death would be about right.