They defeated the dragon and revived the skeleton inside it thinking it was Falin. I guess she escaped the dragon somehow, maybe something startled the dragon and it dropped her and she took the chance to escape. It could be a spell or item in her possession (like a bag full of spices) or maybe it could be something else showing up.
It's funny how nonchalantly badass the Touden siblings can be. Laios is able to make it down to the 4th floor completely by himself, and Falin is able to go return-to-nature survivalist in the same area that a DRAGON is actively patrolling.