Could anyone help me on understanding the following phrase in a novel.

What "touchstone holding him present " means?
What is the condition of David in this scene?

In the novel, David and his wife Laurel found a drowned girl (Molly) in their yard. Laurel tried CPR on her before firefmen come to the yard.
The girl is dead, and firemen take over the body from the couple.

Following it from the novel;

" Laurel bent and pushed a last breath into Molly's slack mouth, and then she knew it wasn't any good. Strong hands came down and lifted her away, like David had lifted Molly out of the pool, and she was passed backward to David as the firemen tookover.
David walked with her(Laurel) to the patio, one hand on the small of her back as if Laurel were a touchstone holding him present, even as the fluid economy of his movements disintegrated."

Your help would be highly appreciated.