Considering you think she is rude and passive aggressive, I would take the high ground. I think you can approach her and discuss it with her privately, or ask to do so at an appropriate time. Non confrontationally ofc. Maybe say something like "Hey, I know you are trying to help and I appreciate you taking my stutter into account, but would you mind not talking for me blah blah blah" If it keeps going after a private chat, bring it up with a supervisor/manager and they can discuss it with them further.