If your neighbour has been referred to the clinic by her GP (or another health professional) they'll obviously want to investigate the condition that caused the referral. Otherwise I can't see how it can be anything other than some form of routine check-up (but I'm confused as to why a specialist clinic should be involved, since GPs and practice nurses normally do that sort of thing).
As your neighbour is clearly happy for you to know about the appointment, perhaps you could ask to see the actual letter? (I'm assuming that you haven't). There may be further information (or a phone number for enquiries) buried somewhere in the small print.