How you act under stress and pressure shows your true merit and tests your conviction to your morals.

You know, that can be true to some extent but it can also be wildly untrue. I think you're overstating everything way too strongly and not acknowledging enough the nuances of individual situations. Context matters a lot and there was a LOT of context to take into consideration for this situation. People are imperfect and say dumb things in the heat of the moment when they can't think of anything else, it absolutely does not have to indicate that "she is okay with that kind of thing on some level." Otherwise do you really believe that every mistake that a person ever makes says something about them on some deeper level?

Obviously it would have been better to strike back verbally in "some other way" but let's be honest here, OP has been accommodating to the max and has put up with a lot, if someone like that snaps it's a bit much to then tell them that they should be essentially in control of how they become out of control. It's like telling someone that they should have known to make a mistake in a better way! At that point you go for the easiest and most obvious weapon, even if it is the most offensive and wrong. It's easy to say in hindsight that someone should just take the higher road, and to be honest, making this about how you would never to such a bad thing (and you've been tested at least once!) comes off as you preaching from a high horse. I suppose I admire you for your conviction of how good you are, but I'd be careful not to fall off in any way. Racism and transphobic barely start the list of offensive things you can inadvertently say but not mean when you are returning an insult/attack...you might find that something you take as ok to say in a return insult/attack is seen as just as morally bad or even worse. It depends on what issues are close to your heart and what you privilege as more deserving of respect/protection.