I am genuinely surprised, I clicked on this thinking "in what world is it okay to not help someone stuck under a barbell" but I stand corrected. I keep rewriting my spiel because I can't properly put into words the ludicrousness of this whole situation. Like... you (1) asked him not to at the beginning, making your wishes clear; (2) installed a lock, which he broke; (3) installed a new lock, which your wife provides him the key to; and (4) provide proof that it's unsafe for him to work out alone, and instead of being horrified that her son could have been more injured (had the weight been higher and you not there), her conclusion is to be mad at you? I mean I feel like I'm basically just restating what you wrote, but like...what more is there to be said? I don't understand how she doesn't see the danger, ON TOP OF the fact that it's your stuff and you've made it crystal clear that you don't want him using your equipment. (Plus gross, he probably sweats on it and doesn't clean it up.)

I think the question isn't whose TA but what you're going to do next (because clearly you're NTA). You took several steps to physically prevent her son from entering your space and she circumvented it behind your back. How do you move forward at this point? My first concern was originally "how do you get them to move out without hurting her feelings" but honestly, maybe it's the wine talking (ok it's a margarita) but how would you be able to trust her again? And do you want this kid to be a major part of your life for the foreseeable future?