Frogs don't need to live in an actual pond all the time. They generally just go there for breeding. If it just appeared there, it will know it's way back to wherever it's feeding/living place is. Just put it outside, if there are any bushes anywhere or in an alley if there is one near the house and it will find it's own way.
frogs are clever little chaps. Shame she doesn't have a garden, frogs are excellent creatures in the slug control department. Has a neighbour got a garden, it could have come from?
hope this helps.