So this is my understanding:
Carla offered OBSG a refund but he declined and said lets do a makeup session.  Carla agreed.
Then Carla changed her mind. No longer offering the session OR the refund.
If I understood the situation correctly, a refund should be made since Carla agreed to having a session IN PLACE of a refund.  Simple business example: It's like if I go to the store, pays for a TV in advance and never got the TV.  The store then offers me a refund, I decline and say I rather wait for the TV.  It never comes - should the store give me a refund then? I think so.  
I do not know OBSG or Carla, just my 2 cents from reading the entire thread. In fact, I'm only visiting Dallas