If you are walking and carrying a shell, REMEMBER, the memorial tree for Denise is located adjacent to the little church / chapel, ON THE LEFT SIDE of the road, just next to the large woodworking factory. This is just before the overpass you walk over a highway on. If you reach the overpass without stopping, you went a little too far.
The memorial tree is NOT actually located at Albergue Ecce Homo, located on Calle Ecce Homo, on the RIGHT side of the road leaving Astorga.
I made this mistake in April. But, I cleared it up once I arrive at the Pilgrim House at Santiago. So, I am sharing the precise location for all, so you are not disappointed.
Of course, you always do have the option of carrying the shell all the way to Santiago, something Denise was, regrettably, not able to do. There, as others have mentioned, you can add the shell to the collection at the small memorial they have there. Periodically, Faith and Nate move the shells out to Astorga to the memorial tree.
I hope this helps.