Whose Fault is This?

So I know this guy who went on a long trip out of town to Thailand or somewhere.  He gave his mother a key to his house so she could check on things while he was gone.  He also gave her the garage door opener.  The key was for the front door, but since he gave her the opener she misunderstood and thought the key was for the door between the garage and basement.  She never actually went into the house while he was gone, just walked around it every few days to make sure the doors were still locked and no windows had been opened and she never used the key.  Eventually she forgot she had it.

Flash forward to recently.  The mother is fostering a dog.  Granted, she didn't think it through real well because she'd agreed to keep her son's dogs for the weekend, but with plenty of crates and a kennel, it would be fine.

The son brings the dogs over (late, so there's no time to take them for a walk together).  The son's female thinks she is the Grand High Imperial Poobah of Dogs and any dog who disagrees must die.  The foster dog, a little pitbull, says, "you're not the boss of me!"  In the course of the son's dog proving that she is, indeed, the boss of her, the son's dog gets a bite in her eye that requires a vet visit.  (she's fine).  Since the son had to leave on his appointed trip, the mom takes his dog to the vet.

She calls the son to let him know his dog is alright and suggests that they keep his dog at his house and she'll go there to take care of her.  He keeps telling her that she has his key and she vaguely remembers putting one on her keychain but doesn't remember which one it is.

The son is still up inside the house packing for his trip when she gets there, but the garage door and basement door are open.  She lets the dog inside and starts trying keys to see which one opens the door.  Since every key turns the lock when it isn't in locked position, she locks the door.   None of the keys work, so she goes back to the car to get her phone and call her husband to see if he has the key.  He isn't answering.  Just then, her son comes out to the garage, shutting the door behind him and locking them both out.

All of his stuff is inside the house.  He doesn't have time to wait for a locksmith.  The mom tries not to laugh but doesn't quite succeed.  She thinks about it and starts to suggest that he break the door knob and she'll replace it that night and that's when the husband calls back.   She gets as far as "maybe you can break..." when the phone rings and she wants to find out if there is a key somewhere before they do any breaking.  The son, who is royally pissed off by now, hears "break" and breaks in his own door and the frame with it, pissing him off even more.

Once inside, he asks her why she locked the door in the first place.  She explains that it was to figure out which key opened it.  He then tells her that there is no key to that door and the key he gave her was for the front door.  They look at each other a minute and she hands him her keychain, with the key to the front door on it.

He thinks the broken door is her fault.  She disagrees.  What do you think?