I 'like' to say just "Thinking of you" on cards like that to a friend. Let's them know that you actually are without trying to find flowery deep words.If you know where the grandmother lives send some flowers to her or you can go old school and send him a sympathy card. People remember stuff like that & it's a nice gesture.
It does mean a lot to them I think.
Edit: Off the back of TSF's suggestion <cough> seeing EZ start with "if you know where the grandmother lives" had me cringing a little inside.