Speculation on my part here, but I think that if the home owner does go down for this, it's simply a case of talking to the cops without an attorney present that puts him away.
Let's go down this road of thinking for a sec. Let's make the supposition that the girl went psycho for a moment. Being lost, on drugs, scared with a recent car wreck, and decides she is going to get into a house regardless. For this mental "what if" assume the girl is actually actively trying to break into a house, but failing miserably at it. Homeowner hears the noise at 3 am or whatever horrible time it is. Opens the door to see what is going on outside while armed. The girl yells, lunges, or does something to spook him for a split second. He does accidentally discharge a shot from his shotgun. Calls the cops and not an attorney right after the cops. He speaks the cops and tells them conflicting information due to his emotional state. Being sleepy from woken up at that hour, disoriented from the mass of cops that descend upon his house because someone was shot, and because he's shaken up because he just killed someone. He tells things he shouldn't in a conflicting manner that can make the police or DA suspect that this was something more than a simple home defense. Especially in an antagonistic area like Detroit against gun owners.
He could very well end up getting charged and losing in court over this because he stupidly said the wrong things during the initial investigation to the police.
I am not saying that the scenario is what happened here. Just saying that at this point it is a very possible scenario that could have occurred.
It could also be he was a cranky old bastard and shot first the moment he saw someone on his porch at 3am.