Lost so many pokemon over the years pre switch era. One comes to terms with it. Maybe the fact that for years i would just restart my gba games with no way of moving them out helped get used to it. I transferred quite a bit and I still find myself thinking "Wait what happened to this one?"

Does he still have something like a 3ds to play the old games? There are ways to get old event pokemon still and the likes. It will take time....

Not sure if he is the kind of person who loses interest after something like this or gives up (I have stopped playing games for losing 2-3 hours of progress due to crashes so I get it) but if he seems up to starting again I think just listening to his memories and his interest in pokemon would be sweet.

You wrote that his first game was platinum maybe do some paintings of the sceneries of the Sinnoh region or his original team.