If someone has attachment trauma they need a therapist who is not trained in attachment therapy (which has been around for ages and has had dubious success) but one who is trained in the neurobiology of attachment trauma. That is an entirely different thing altogether and is a more recent area of study.
My ex T was trained in this. She was detached but supportive and very aware of even my most subtle body language etc that signaled the attachment trauma was activated. Most sessions I left her office fully grounded in the present and clear headed. The goal of attachment trauma therapy is to keep the client within the window of tolerance both in and out of session so that the attachment trauma is able to be self regulated. this is very different to attachment therapy, which activates the attachment and then abandons you to your own mental wolves.