the h/d kits are rated to 2100kg braked,
there was a square hitch 1600kg kit too though, but there will be a sticker on the inside of the drivers door jam, and on the towbar ontop of where the hitch slides in,
in saying that though, i wouldnt go towing something heavier than the car anyway, unless it was slow and around town, not over 80km/h
There are a few points raised in this thread so far.
In relation to the OP, the legality of leaving the tow bar tongue attached to the car is not an issue. It was legal and the old bloke was pissed because his mussus whacked her shin. As people age, they become less tolerant, so in his mind, you were at fault, not his wife. The elderly seem to think that everything they don't like is wrong in some way.
As for protection against this sort of thing happening to the elderly or anybody else, then yes, removal of the tongue, or installation of a shin-guard are practical steps that can be taken. I remove my tow bar tongue when it's not in use, because of the danger to my legs and others, but also to give me a bit more room in the garage when the car's in there and to enable the auto door to be closed. (Tolerances are a bit tight with the bench taking up a few feet of the garage depth across the front of the car).
Knocking off the shin guard is a bit difficult - they should be firmly trapped between the tow ball and the tongue, so helping yourself to somebody else's wouldn't be a simple matter.
In NSW, as far as I am aware, it's illegal for the towed weight to exceed the weight of the towing vehicle. I've often wondered how anybody can legally use Holden's 2100kg towing set up here.
And finally, in NSW it's illegal to be in possession of any item of Police uniform if you are not a Police Officer and I'm certain that this law applies across all States, so I'll assume that the OP is on the right side of the law and has a member of his family who is a serving officer.