I will admitt to 18 years in retail clothing in management & as a
buyer...

 Having said that...Where was the sales clerk when yopu were making choices?
 If she had appeared & helped ypou into the dressing room, as she should
have, then she would have been aware you needed help & either arranged it
for her to help, or allow your father in.
    Her attitude was terrible, and I think defensive of someone caught not
doing their job properly.
I would definately write management, and if you don't get a response...A
letter to the editor of the local paper might bring it to light..      Joy