I think he just disappears.(i haven't played diamond lately.)
either dialga/palkia puts him in a space/time rift or he just goes to sulk somewhere it only says for sure in platinum
After you have the 7th badge, go into mt. coronet. Keep going up, and battle all the team galactic morons who wanna battle you. soon youll see a cave. go in and beat two team galactic commanders and the galactic boss. then dialga is waiting!!!
you go and battle dialga
once you go through mt coronet's summit (beating galactic's boss and catching the legendary) go to sunyshore city, via route 22
once you reach the top i had trouble to but on the left go sround and on the right side on top of it all its a small indentation
you need to go to the other side after you beat the game
Battle them. If they don't battle go another way.
the 1st time u go through mt cornet u meet him
Try to find your way to the boss and battle him. Then you can go through the door in Mt. Coronet.
no where cause there gone to a different place
After you beat Team Galactic's Boss in their Veilstone City Headquarters building, he'll go to Mt. Coronet's Spear Pillar in Pokémon Diamond, Pokémon Pearl and Pokémon Platinum so that is where you'll have to go after beating him in his Veilstone Headquarters as well as after freeing Uxie, Mesprit and Azelf from their trap containers that they're in.
You have to go to the top of mt conoret, where you'll have a battle with your rival against commander Mars and Jupiter. After that you'll go to the distortion world where you have to beat Cyrus and fight Gratina. Once all that is completed, team Galactic will be gone from mt Coronet. To find Cyrus and the top of mt Coronet: http://www.mahalo.com/Pokemon-platinum-walkthrough-part-6/
You go to Vielstone city and go into Team galactic building and find Cyrus(Team galactic boss) then battle him. After the battle he should give you a master ball.