The cop would say "I felt threatened" end of story
The way you put it, it sounds like evolution in action. There are times that stupidity can be lethal.
The problem is, you don’t have a right to pull a gun on just anyone that happens to be in your home and give them orders to be obeyed at their mortal peril. There are rules as to when you have the right to do that, and when you don’t.
Since the boy was an invited guest, the father didn’t have that right, even if the father wasn’t the one who invited him. The daughter’s lie may have led him to believe that he had that right, but I don’t know if just that belief will protect him from prosecution.
In the end, the father’s lawyers may have to make a case of self-defense, but that is a higher standard and they would have to try to show that there was a reasonable belief that he was in danger, from a probably naked and unarmed teenage boy.
I think if them daughter and father had enough time to converse about who this interloper was, and they boy was standing there without an escape route, the father is looking at premeditation. He will be indicted, and probably convicted of second degree manslaughter. The fathers response is predictable and understandable. I do not see a winner in this situation.