If he uses the Catalyst graphics driver, he should try Set Catalyst AI on and start the game in compatibility mode for Windows XP (right-click the game's start icon => Preferences => Compatibility tab).

He could also try lowering overall video settings in the driver's options panel as well as in the in-game's Video Options. to see if it works with everything turned off. If so, turn them back on, one after another to identify the problem.

Also check if he's using the newest drivers or, if you want to take the effort, download legacy versions and try several old drivers.

Transparency of walls could also be a texture issue. Does he use any texture mods?