As long as the baby is not already dehydrated, the fact that they still have formula in their stomach means they still have the water in that formula in their stomach.
But as long as it doesn't affect the amount she is eating the little bit of water won't hurt her. I know how crazy with worry these guys can make you and like I said, if you are worried about this, by all means, give a little water. Just don't affect the amount of formula she is taking.