Also, they can just declare death by incantations but they die as well so I suppose that's smart of Beybey to just use an old fashioned knife :P Honestly makes me kind of worried that this detail was even mentioned, because there's always a chance it will come into play at some point...
Assuming she actually understands that her instakill curse is a self-destruct move. If they're treating her like a disposable weapon anyway, there doesn't seem to be any practical reason to tell her what the consequences are. Just feed her a half-truth about it being single-use so she doesn't use it early, spin a yarn about welcoming her as a hero on her return, and now your weapon disposes of itself.
(Also, I wouldn't be too worried. The point of introducing it might be to convince Beybey that even if her tribe were still around, they never cared about her anyway. It's already been a handy way to get the protags involved, raise stakes, and give them an excuse to adopt, so it might've already run its course.)