I thought you were saying FutureShop sold him a PC in that condition. Now I gather you mean he took the HSF off himself, and left it off. Leaving the card without the provided cooling solution would definitely give FutureShop the right not honor the warranty.
However, since you brought it up, (and because I just had a similar discussion on another site) I will note those stickers mean nothing! Users have the right to open the side of "their" case, upgrade RAM, add a drive. More importantly, they have the responsibility to keep the interior of the case clean of heat trapping dust and dirt. So makers can put stickers everywhere, it does not matter. As long as there are "user serviceable" parts inside, you have the right to open it up, and do whatever maintenance you need to do - as long as it is done correctly. If there are no user serviceable parts, such as inside a hard drive or power supply, then breaking the seal might void the warranty of the drive, or PSU. But even still, in a court of law I suspect they would have to prove that breaking the seal damaged the device, or that something inside could only have been broken by abuse. Just breaking the seal and looking inside may still be within your rights. It is your PSU.
Understand that most independent PC assemblers, including yours truly, feel those stickers are nothing more than a scam to coerce and scare owners in to having all maintenance, including upgrade purchases, done through them. Consumer protection laws protect us from such abuse. Automobile makers tried the same thing and were shot down long ago. You can do all the maintenance (routine and otherwise) in your own backyard, as long as you do it correctly, using any oil and parts that meet industry standards, and your warranty is still good (just keep your receipts for supplies). The big electronics firms tried that with TVs - yet tube testers were in every drug, grocery, and hardware store because we had the right to break the seal, remove the back, pull the tubes, and take them for testing.
So if someone says you broke the seal, the warranty is void, remind them it is your machine, your right and you will gladly meet them in court if they choose. Since they know they will lose, then get stuck with the legal fees, they will honor it.