Steam Dota 2 Issue

He said it doesn't appear on task manager or in background processes.

That's simply impossible. Tell him to check the details tab in task manager (if he uses Win 10).

I had similar issue in the past, but with another game. My guess is that it's a steam bug since I found a soultion online. I think logging out and back into steam account helped, but I can't remember properly. Could you tell him to try that and see if it works?

I don't get what you are really saying... so when they start their windows and opens steam it just opens dota 2?
re-login, reinstall steam, update steam, update dota, validate missing files, trouble shoot, all of these might help tbh

it launches itself but not actually opening and is not in the task manager too,