Hi there.
This is just my opinion:
Sounds like she is addicted to shopping. I believe I read somewhere that even though she is returning everything, it means the same thing - like people who shop and fill their houses up with so much junk that still has the price tags. It's good that she is returning it, she probably finds walking down the isles relaxing and then when she gets home realizes what she's done and returns it all. There's a good chance she notices what she's doing too but just keeps doing it.
Idk, kind of think of myself with this - I used retail therapy a bit too much, it became as common as having 'one more cigarette' just 'run to the store for a minute'. Only, I didn't return the junk I bought. Wasted a lot of money. ...
I hope you can find out a way to help her. Whatever the reason for her actions, I hope she is able to alter this shopping issue.
Best of luck.