its just an appointment system though...you cannot view any information about him, nor he about you, other than the clinic name - which is obvious anyway, and your name and address - which is not a secret and can be seen in any phone book...
if she is expecting some sort of cash for this then shes got no chance - no harm has been done....
i would however flag this up to the clinic etc - and they clearly need to be more careful - presumbaly the receptionist just put the letter in the wrong envelope - so its likely a one off...