OCCUPY PET SHOP. <-- best idea, ever.
My local fish store is also killing their fish. I am not physically able to do much, but I have left a nasty (and truthful) review on the store on its google listing page, and intend to call the owner to tell them how the sick bettas in tiny cups make the store look bad. If they respond rudely, I'll call the BBB and RSPCA. Maybe even PETA. ;)
Since the owner of your store is belligerent, a phone call or two to several authorities might help.
Have one of your friends (since he knows your face) dress in their best job-interview clothes, and walk around with a frown, a camera and a notebook, saying nothing at all to the owner (in this case, have her walk right in, snap a pic of the bettas, frown a bit, take a few notes and walk out). It unnerves bad pet store owners badly. I have seen stores start to keep cleaner cages after doing this.