Technically, she can. But it is very high handed of her to rule that this would harm you, when she is no longer your treatment provider and (I feel) cannot make that decision. That decision should be made by your current T, and if you can get him to agree that it would not harm you, and to write a letter to this effect, then she would be on very shaky ground if she tried to overrule this.

I agree with others though that it sounds like it might be damaging and counterproductive. I feel you should obtain the records, now that you've started (on principle!) but make the decision about actually reading them separately.