Yeah, this is the point I was trying to make (along with what Elio said). The thing a T can provide that's different from a friendship is space to fully explore the awful rejected feeling and figure out what to do with it. That's not the same thing as being comforted or reassured, either.

This type of situation is actually exactly what I meant about therapy being a laboratory. Something happened interpersonally that feels really, really awful. Can you stay and figure out with the other person what the heck happened? Can you repair the relationship in a healthy way? Casual friends can't/don't do this sort of conflict resolution thing very often, but good friendships and strong romantic relationships can do it when the need arises. Therapy is a place to practice dealing with intense stuff in a sort of "training wheels" way. You can mess up, express your anger is somewhat unproductive ways, say things that you don't quite mean, and still somehow come back from the mess. A good T has a relational stance that makes that possible. They can care about your feelings and your pain while ultimately making it all about you.

The thing here, though, is that I don't think DP's T understands any of this. If he can't do the therapeutic work here for whatever reason, he isn't a safe option to be a therapist. This is just going to get more complicated and painful until it explodes in an even bigger fireball. I'm not just saying that because his shifting boundaries created the situation (although that's a pretty big red flag) but because he already sounds like he has no clue how to work through the situation.