Nonlethal weaponry - for defense of others
My question is situational, and the situation may come very soon. To better help you help me, I'm going to tell you a little story. Feel free to skip it.
My girlfriend's mother's taste in men ranges from bad to absolutely, unbelievably terrible. For the past six years she's been in a tumultuous relationship with a man who, for the purposes of this thread, we'll call "Joe." Joe is an idiot and a drunk who primarily sustains himself by preying on other people's kindness. For these past six years he's leeched off the mother, unwilling and/or unable to hold down a job, perpetually owing the mother money and giving her just enough affection to satisfy her overwhelming personal insecurities and make himself wanted despite his incredible worthlessness. Joe also has anger issues, and when heated words are exchanged (and they often are), one can safely wager that he'll be quicker with his hands than his wits. Though the mother is in her late fifties (as opposed to his early forties) and much smaller, Joe, sober or not, is more than willing to strike her. Thankfully, the mother's son (who, having moderate schizophrenia, lives with her at the age of thirty) is a large man who takes great pleasure in ground-and-pounding the good-for-nothing whenever he steps out of line. The past six years have gone by in a fairly predictable routine: Joe acts up, attempts to strike the mother and is subdued and beaten by the son.
Fortunately or unfortunately, the son has recently had a change in schizophrenia medication. Whereas he once would sleep all but three hours a day (unless roused by my girlfriend to give Joe an ass-beating), he's now active for almost a full twelve hours, most of which he has chosen to socialization outside of the house. This is great for his social life, and certainly great for his quality of life, but unfortunately, since the son is as often outside of the house as he is inside, Joe's natural predator is gone.
My girlfriend isn't worried. She views Joe's violent mood swings as the natural consequence of her mom's bad decision-making. She knows that her mother is psychologically dependent on Joe (she's put up with this same **** for six years, after all) and unwilling to make the right choice, so she waits for Joe to land a blow and calls the cops (if she calls the cops before Joe gets violent, her mom takes his side and the violence is merely postponed to sometime she isn't present to phone the police). I am her opinion - that the mother is willingly bringing this upon herself and is responsible for the consequences - but if the situation comes, I can't stand idly by and allow anyone around me to be beaten.
Which leads me to my question. I have but five months of boxing training and three weeks of BJJ training under my belt, certainly not enough to subdue this average-sized (probably 5'10" - still larger than me, a wee man at only 5'7") but very strong man with high pain tolerance - not, at least, without taking significant damage myself. What can I use to stack the odds in my favor? I doubt the police would allow me to blow his brains out or spill his guts open (though the world would be better for it. He's the only man I know personally who I can say that about), so it'd have to be nonlethal and preferably non-reliant on pain compliance. What's more, it'd have to be very well-controlled - I wouldn't want the mother to be caught in collateral damage. There would be a window of time to prepare for an outburst of violence (just enough to wrap my hands), so the weapon doesn't have to be insta-deployable. Most of these incidents happen over one or two rooms of the mother's very cramped house, so it'd also have to be effective in very close quarters.
This is all hypothetical, as no such unmitigated event has ever occurred, but an argument between Joe and the mother very nearly spilled over into violence yesterday and the son wasn't present to defend her. Any advice on a weapon to use or tactics to employ would be appreciated. I would much, much, much rather be safe than sorry.