Help me out!!

Taking my baby to the babysitter for the first time tomorrow I am conflicted on preparing the bottles for a babysitter or having her thaw the milk on her own but that’s besides the point I have a huge question.

Say she prepares a 3 ounce bottle of warm milk....under warm water and the baby drinks one ounce what happens to the other two ounces?

Can it go back in frig?! To be warmed up again? Time limit on the milk?! Throw away?!

I read different things.....