I think that whether or not CE needs to accept responsibility for his role in the group's demise depends on the therapist's stated expectations for his behavior at the time, not two years later. If he was led to believe that she could handle his anger and that it was important for him to have the opportunity to express it verbally no matter how hateful it was at that point in time, then he should bear no responsibility for playing a part in her decision to end the group.
If I do something that my therapist cannot tolerate and it is affecting his decisions, he does not wait two years to tell me, "Hey, by the way, remember when I made "X" decision? Well, that was partly because your behavior contributed to it." My therapist would find it grossly unfair to hold me partially responsible for a decision he made without making it clear to me at the time that my behavior was having a negative effect on him. I am in therapy to learn about myself and my destructive patterns of behavior, and it would serve no purpose for my T to act like my behavior was tolerable to him when it wasn't. So he is very clear about what is acceptable and what isn't.
If CE's therapist had said at that point in time that his behavior was contributing to her possible decision to end the group, he would have had the opportunity to change his behavior before she made her decision to end the group. If she did not make that clear to him at the time, and she led him to believe that all his anger was tolerable and it was important to work it through, then it's patently unfair of her to blame him at all for her decision.