LTL;FTP, mobile, all that jazz

Hello reddit! ^~^
Context first: in my main friend group, two of my friends are dating each other. We'll call them Alex and Bradley. Bradley and I have had issues with each other in the past, usually stemming from the way that he treats other people. According to him, he has a disorder that converts stress into pain (conversion disorder, for those interested). Because of this, he usually refrains from feeling/expressing any emotions, even those outside of stress. This wouldn't be a problem if he could figure out how to still be nice to people, regardless of how he feels (or doesn't). A good example of this would be that at one point, he made a joke to a more distant friend about selling nudes of another close friend, we'll call him Jack, in exchange for borrowing a calculator. When Jack found out about this joke and was understandably upset, Bradley continued making jokes. It took three of us ten minutes of telling him that his actions were inappropriate, he hurt a Jack's feelings in a serious way, and that he needed to apologize, for him to finally give a measly "sorry bruh" (direct quote, by the way). Issues like this happen a lot of the time, and almost always involve him refusing to understand that his actions are hurting other people (even when directly informed)

Now, on to the main issue.

I consider myself and Alex to be close friends, and they have told me they feel the same way. In Alex and Bradley's relationship, Bradley's refusal to recognize when he is hurting others causes frequent issues. When that happens, Alex will often vent to me. The issues range from accidental hurt feelings to deliberate, admitted manipulation. Alex has BPD, which makes them what most would consider "high maintenance" as well as easily hurt, and easily manipulable. Bradley knew about this before the two of them started dating. Bradley also thinks himself to be an expert on any and all mental illnesses, to the point where he says he "cured (his) own depression but then gave it back to (him)self again because it was boring not being depressed". He tries to cure Alex of their BPD, manipulating them using the silent treatment, among other things, to try to basically train them out of their behaviors. I have BPD as well, which is one of the big reasons Alex vents to me - I understand their point of view better than an otherwise mentally healthy person would. Bradley feels that Alex is too sensitive and impulsive because of their BPD, and is trying to fix them. I feel that what he's doing is hurting them, and he isn't a licensed therapist, and so he needs to just be more considerate of their feelings (or end the relationship if he can't handle dating someone like Alex)

Now, I know if I were just shoving my nose in out of the blue, I would be the asshole. Toxic relationship or not, it wouldn't be my business. However, before doing anything, I asked Alex if they wanted me to intervene, and they said yes, and that they appreciated having someone try to defend them. They also said that it might help Bradley to understand what he was doing wasn't OK if it was coming from an outside point of view. 

Bradley, however, does not know that Alex is OK with me involving myself. He sees it as me sticking my nose into places it isn't wanted, and has started treating me somewhat badly because of it. I can clearly tell that from his point of view, I am the asshole. But from mine, he's the asshole. So, reddit, what do you think?

tldr: Two friends are dating, pretty unstable relationship. One asked me to help try to keep the peace between them, the other sees me as rude for sticking my nose where it doesnt belong. AITA?