To save someone from falling indicates that they have not yet fallen. The use of "from" is important, as it indicates preventing an action before it happens:
It would be the same with any similar action:
He stopped her from drinking the old milk.
She turned the wheel to keep from running off the road.
He forbade them from playing in the rain.
And so on.
If instead you wanted to say that the girl already fell, then often the past or the past perfect are used:
He saved a little girl who fell/had fallen into the water.
With a different grammar structure you can use "falling", but with some adverb like "after":
After falling in the water, the girl was rescued by a passing stranger.