I'm so glad this thread didn't explode with opinions about piracy and copyright in general.
OP, you have three scenerios:
1. Buy the game. $10 is hardly a ball breaker and at least you know it's clean.
2. Download the disc and find out fair use protects you and no one says anything and you play the game.
3. Download the disc and find out you just broke the law and no one says anything and you play the game.
I'm personally an advocate of option 1, since it leaves no chance for questions. As for 2 and 3, there is a "right" answer but no one here can give it to you. If anyone would actually read the Digital copyright act, it specifically mentions that a copy of the original physical media can be made for backup purposes (these were the floppy days after all, and they went bad often). The law only protects your right to make the copy of the original media, not your "ability" to make the copy, which is why it is illegal to circumvent DRM. The problem with this is that if he downloads it from the internet, that copy wasn't acquired in a way that is acceptable by the law as written, thus the ambiguity. I'm not saying it's legal or not...only that the law doesn't deal with this scenerio. My opinion would be that downloading it is within the spirit of the law and thus legal, but that opinion means nothing.
Unfortunately, none of us our federal judges and therefore we can't rule on the "intent" of the law. Until they revisit it and update it for todays digital world there will be people on both sides.