Until she is deemed otherwise she still has capacity and so is entitled to doctor/patient confidentiality so I can see why the doctor might not want you there, likewise, they may feel having someone present could influence them just by their presence.
I've always attended all the appointments and test with both my mother and wife both of who had/have AZ and no one has ever objected, however, we have always been separated for at least part of the appointment and spoken to alone.
The phrase "capacity reassessment" doesn't come up very often, I take it there is some dispute over her capacity if so that would be a good reason (to me) to want neither side present as even the presence could be seen as an influence on them.
K