I say you should set up the 1 gallon bowl for her and put the betta in there when she isn't looking. Then leave it at her house so she can realize that bettas aren't toys, decoration, or whatever! Show her bettas deserve more! Plus, I do not like stealing.
Yes, you should try that. And if that doesn't work, then take it. Besides 1 gallon bowls still look nice if properly decorated much nicer then that vase anyways.
hmmmm.... maybe show here pics. of other fish in small "vases"... maybe that will get her to listen... or surprise her and set up the tank for her and slip the fish in.... anything but that tiny,tiny,tiny little vase