A mental health therapist can and should be able to help a client manage their righteous anger as they work their way through it together. Anger underlies a great many things, and grief is one of them. You don't just say, "I don't want to feel angry," or just decide, "I'm not going to be angry about this anymore," when the rage is this broad and deep and well-established: in time, that kind of denial and repression is just going to explode in your face, if it works at all.
Instead, because anger is very well understood, there are known ways to deal with it. Anger doesn't just disappear on its own, nor even very quickly, and a person suffering this kind of grief could use some assistance diving into the anger and the very exact specifics that are causing it. The therapist should be able to navigate this presenting anger with relative ease and guide the client through it, not just with skillful therapeutic inquiries to get to the very specific root of this writer's unique grief which is where they'd start, but also with coping skills to give the client to be able to self-manage their righteous rage for as long as it takes them to work through it and clear it so that their overwhelming grief can resolve.
This therapist is doing none of that. Instead, they are just offering commentary on the presence and length of the anger as though they're watching a movie, when they could and should be helping the writer sort it out and work through it. That's not on the writer, that's on the therapist.