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This guy already legally owns the game and uses his own serial key to install it, so it can't be considered piracy. Also, no links to cracks and warez are being posted (and so far you're the only one who mentioned a crack; although FAFAIK cracks are legal as long as you legally own the game), so I don't think locking this is necessary
Indeed, I have done that too, and I don't consider it stealing; I'm just using TS standalone instead of part of TFD, for it is more practical for modding. Your serial represents your license, not your CDs. However, I'm not sure whether it still could be considered somewhat illegal, but there's no difference in downloading backups and making backups (which he in this case hadn't). Therefore, it can be considered on the edge of legal and illegal, and I think it can be tolerated.
On the other hand, you're best off burning out back-ups of the CDs, and doing as Blackster told.
Perfection can never be achieved; it's merely approachable.