So recently, 2 of my friends have experienced really bad artifacting in custom maps, and they both have the same card: GTX 660. Would this be a driver issue or something else entirely, because they say it gets to bad to the point they can't see anything in the game anymore.

No, it sounds like the issue many other people are running into with some custom maps, (Requiem, Project Viking, etc.), tell them to try disabling Specular Maps in the Graphics Options. Make sure to restart game also when trying.