I am so sorry. Give difficult child a hug for me, if he will accept it. Sending them to the entire family.
Having her on the country property will be a help in later years. You might let him make a marker for it. Then he won't get upset for "losing" where her. With the exception of gfgbro's dog who was killed in Idaho, all of our animals are around a tree in my parents' yard. I know that Wiz sometimes still goes to talk to Freckles out there.
You can get a kit to mold a marker for her. Or if you have a silicone cake pan or even the bottom of an appropriately sized cardboard box you can use that. Line the pan with several layers of newspaper, making very sure that all the sides and corners are covered. Cover the newspaper with 2 layers of plastic wrap. This is to protect the pan. If you use a cardboard box, just line it with newspaper to keep it from leaking out of any corners or seams. You can just cover them with package sealing tape from the inside if you want. Or use an old cake pan that you don't mind throwing away.
Then go to the store and get plaster of paris (can go get cement mix from Lowe's or home despot if you prefer that). Mix the plaster as directed, tint if desired - if you have tempera/poster paint you can add that to the water or the wet plaster. Stir in thoroughly. Pour into pan/box/whatever and then see if whatever rocks, decorations, etc... stay on the top or sink. If they sink, let it sit and check every 30 min or so until the decorations sit in the plaster but don't sink.
Decorate, let dry for several days, and pop out of the mold. If it is in a cardboard box you can just rip it away. If the bottom of the box stays attached it won't hurt anything. Silicone cake pan can be just pulled off. Or if you don't want the cake pan, just leave it on. Then give the marker a few more days to fully dry. After that, bury it level with the ground in the appropriate place.
FYI, try to get the marker 1 1/2-2 inches deep so the stone will last.
(If you have extra plaster that is tinted (or that you are willing to tine), it can be poured into empty toilet paper rolls or molds for jello/candy. After it hardens it will work as sidewalk chalk. Plaster of paris is best for this.
I know it was a big help for my kids when we did this, and it might help difficult child process his grief also.
