Once you have deleted the game, by what ever method,
when you open stream game list and click on AC to play it will prompt you to re install.
He seems to have done that already.
Ha! I just had an idea; I reckon you can try this.
First; copy all downloaded content (steamapps/common folder) to a safe location so you don't have to re-download it all.
Make sure you delete the AC related things; (you can of course try and copy the steamapps part off as a backup to save the d/l again)
- steamapps/common/asstto corsa
- documents/assetto corsa
Then uninstall steam, restart computer, reinstall steam
Then copy the steamapps back. When you try to play a game it'll act as if it's downloading the game, but it'll go very fast - it basically checks the local files against the online ones, downloading anything that does not match.
At this point you can either try it with the d/l'ed AC folder, or a virgin download.
I'd also contact steam support in the meantime, they take notoriously long to pick up a case.