If that's the case, he must feel terrible.
Still, I find "I thought it was an intruder!" accidental shootings to be one of the
stupidest ways to die.
In the US, you have to prove you gave the intruder the option to leave before you blow them away. Either verbally or by barricading the doors, showing them you know they're out there and that they can go away and not face consequences, if they wish. Otherwise it isn't self-defense, it's homicide. If she'd lost her keys, she could have knocked the door or tried to call him instead of using the window (speaking hypothetically). And if she did knock on the door and he blew her away either without asking who she was or noticing that she wasn't home, he's so rash that he shouldn't have a gun.