The disc is inserted in the drive. All other games he has work fine. This is the second copy of Sims3 this has happened on, so it shouldn't be a disc flaw. I've seen other reports of the same problem, but the only one that claimed to be a resolution (on a board called "Vizzed") doesn't seem to work.
EDIT to Clarify: There has been no attempt to copy the game. The DVD drive is not damaged; it works fine with other games. This game ONLY has problems.
Any suggestions? We were previously able to play this game on this computer, then one day it just started giving this error. We assumed that the problem had to do with the first disc going bad, but now that's obviously not the case.