I read it as the standard "repressed childhood memories of an abuser come back under therapy as a adult, parents are blindsided and side with their trusted friend over their adult child" which is sadly common. But even if they had no idea until recently, it's still crappy that they're not supporting their kid. (Even if they have actual good reason to believe it's false recovered memories - which are still probably more common than most people would like to believe, because human brains are bad at their job sometimes - their kid is still in a lot of pain! That should their first priority here and it's not!) If that's the case I'd say it's reasonable to cut them off, but also reasonable to carefully keep contact, and probably worth thinking about very hard and not just making a snap emotional call (though even then probably not in the exact terms the therapist is suggesting here.)

But you really can't tell from the letter - it could be anything from that to the parents themselves were routinely committing severe abuse and should be kept five miles away from all children at all times.