I disagree. If she's not going to take care of him then she doesn't even need to think it's her fish. That concept could work on a 5 year old because they are not old enough to understand.
On a side note, I still can't believe my parents let me have a betta when I was in like, 2nd grade. Neither of my parents ever even helped care for him... He actually died under my parents care when I was at camp for a week (not because he was sick: they let my cat get to him.)