First and foremost, without the full story what you wrote here sounds absolutely terrifying that it is about your therapist. I believe you had a negative experience with this therapist. I want to add that your experience is your experience and the fact you are still angry about her in some situation that you were not present says those words or that story touched a nerve for you! I honestly hope you focus why this story touched you so deeply, so injustice, so raw for you rather than how bad the therapist reacted or felt about that person. Switching the frame that way and making it about your feelings and your take on this story is more important but also much harder than focusing on her and raging on her and punishing her by shaming her, and by showing her you know she is not a good person or therapist.
Did you ever feel those feelings in the past?
You will write a letter while you are raging and angry and out of body experience and solely focused on her and then eventually, you will calm down, feel grounded, get back into your body and may feel completely different. You may feel so much empathy and sadness for the other person. You may wonder if your therapist was saying those words while she was dysregulated (let us not assume all therapists are gods with no issues of their own), or that maybe you were feeling down and touched by this story much harder than it may have otherwise…million different ways.
In short, by writing and being sarcasm and trying to shame her says more about your state of mind than hers. If you would have said something right there and then when the conversation was going, would have been more satisfying. Now it just seems petty and revengeful and you are trying to hurt her.
Because she was/is your therapist, I would say, personally, I would not be surprised if you are in transference and displacing some real hurt and anger from the past. That does not mean she is gold. It means she had similar way of approaching you and you are ready to defend yourself, but you are not there anymore. You can see her and calmly explain that story was not called for and you felt offended and hurt and you felt empathizing with the other person than her. Talking like this to her would probably hurt her (in a twisted way) than you write a letter and it all comes down to ya! Fadeway is dysregulated again. No. Fadeaway did not agree with this stupid story and she can tell you verbally and calmly without losing her ground. But if you are losing your ground, then that is an indication, this story touched you deeply and that is OK too.