Have you just considered ignoring the doctor's receptionist. We all know they can be a bit obstructive and self-important at times (most of the time). Your mum is 82, there's bound to be something she needs to talk to the doctor about, just make an appointment and on the way out say "oh btw would you mind signing this photo of me"?
its not that simple because they need to fill in part of the application and include there passport number
