It may not even whine in his system, its comes from the power regulation, so on his set up it will probably sound different or may even be silent (a different power supply could fix it for you too but its not guaranteed to stop it, might just make it quieter, or could even make it louder which is why i didn't mention replacing the power supply)
It won't reduce the lifespan or be harmful in anyway, unless its a capacitor, but that will become obvious very soon as it will fail before the warranty period (at which point they can't deny giving you a replacement), but thats unlikely since its already made it 4 months
If it is a capacitor though, he can just give the card back to you and you can just use your warranty to get him a replacement so its not a big deal
Just make sure he knows it has coil whine and explain what it is if he doesn't know
Let him test it in his system and see if he's fine with the noise