What is the meaning of "touchstone holding somebody present" in this phrase

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.

The discovery of the girl's dead body has been a great shock to him. When this happens, the body has physical symptoms, such as a kind of shaking (like a kind of shivering) -"even as the fluid economy of his movements disintegrated." That is, as he walked, the usual smooth movement of his limbs was gone, and it suggests his legs felt wobbly as if they might buckle under him. His body didn't feel 'real' like he was accustomed to it feeling. He had his hand on the small of his wife's back as he walked. This means, as well as feeling these symptoms in his own body, he could feel his wife's body...and it felt solid and real. As long as he could feel that, feel his wife right there, solid and real, then it meant he was right there with her, there in the yard. He was not going to flip off into some crazy unreal world in his mind - this really had happened - they had found a dead body - it was not his crazy imagination - and the feel of his wife's body was a touchstone, a yardstick, a criterion of reality that he could use to keep himself grounded and know what was real and what was not real.

To David L.
Thank you so much for very detailed explanation!!
I really appreciate your help.