After Hale disappeared while examining faraway ruins, David was informed of this by Hale's assistant Schuyler. They were discussing the events when Hale's daughter Molly came into the room, and David volunteered to give her the news.
He was later seen when Molly first unleashed the Unown, and was forced to flee to the local Pokémon Center as the mansion (and later much of Greenfield) became crystallized.
After Ash and his friends intervened, Molly abandoned her fantasy world, which returned Greenfield back to normal. As a result, David was among those who returned to the Hale household. In the ending credits, David was still seen working at the mansion after Professor Hale's return.