If you want the marker to stand up, you need a stand that won't deteriorate in the ground. If you want it to lie flat, a stepping stone is a good idea. You can make your own by using heavy cardboard and duct tape to make a mould of concrete. OR, you can use the method to which I've become addicted - diamond lath/concrete. Picture of one I made for a friend follows: The picture that comes up is the back. Check out all views to show the process. (I know - I'm shameless - any excuse to show my stuff).

also-you could make a mold of a cross and insert either a copper pipe or some electrical conduit pipe in the concrete as you pour it to have a pole to stick in the ground. Either way would be lovely and a nice thought for your friend.

I have done what WW said it worked out great .