My friend is having some trouble..

My friend and me finally got a beta key, so we played gmod 13 for a while.
I told him to close his game, and he did, but it still says he’s playing gmod 13. (no, he’s not trolling me.)
But he cant gmod 13 but it will say “Please exit any running games or tools befor attemping to verify your game cache”
Is there a way to fix this?

Task Manager, check if hl2.exe is still there. If not, close gameoverlayui.exe from Task Manager, if that still doesn’t work, close Steam from Task Manager and start it back up.

It worked. Thank you!