Well, not a betta, but I have a gourami in QT with some cardinal tetras (bought from a store with central filtration). They're fine, in fact the gourami has decided he's a tetra and schools with them.
I would keep the other tank set up and keep an eye on them for a while, just in case.

I would keep the other tank set up and keep an eye on them for a while, just in case.