PSU has 8 pin slots, GPU has 6?

You need the opposite of what that cable is - an 8 pin to 6 pin cable.

That being said, the 8-pin cables SHOULD have two of the pins removable.

Actually, I just checked the PSU specifications, it has one 6-pin and one 6+2-pin.

I think you guys might be looking at the CPU header, not the PCIe header.
 
Is this his PSU? http://www.newegg.com/Product/Product.aspx?Item=N82E16817341018

If so, it says he should have 1 6 Pin PCIe connector, and one 6+2 Pin - which works just fine in a 6 pin slot (you just don't connect the extra two pins). Have a close look to see whether all 8 pins are moulded together, usually the last two can be separated allowing it to go in etiher a 6 or 8 pin slot.

If it really is ONLY 8 pin, which is pretty strange, that one you've linked wont work because it's the wrong gender. You need something like this: http://www.newegg.com/Product/Product.aspx?Item=N82E16812423178&cm_re=8_pin_to_6_pin-_-12-423-178-_-Product
It has a female 8 pin connected to take the 8 pin from the PSU, and gives you the 6+2 Pin to go into the card.

Usually adapters on power cables are a bad idea, but this is technically fine. All your doing is take an 8 pin cable (rated for 150W) and using it in a 6 pin slot (rated for 75W).

I'm not so confident in the quality of the PSU... your friend is probably taking a bit of a risk with it.
 
Connector_mbpow_pcie_6p2pin.png


Ya plug the 6 pin in, leave the 2 dangling