I dont know what computer he has specifically. It is an E-machinces with a integrated graphics card at the moment. He has a 350w power supply but ill probably install a 600w cooler master for him when he buys a new card. The two cards he is deciding are
For your friend's computer, emachines are pretty bottom line. But then again, he's only playing half-life and that doesn't require much at all now days.
Out of the two I would go with the GT 520 because it has DX11 and is probably more effective with energy consumption.
If you are going for the 520 you won't need a 600 watt power supply. You could do just fine with an antec earthwatt psu rated at 380 watts it is also a 80+ bronze, which means that it is much more efficient than lots of other cheapo power supplies out there. It cost about $40.00 (us dollars) with free shipping.
Antec has another power supply on newegg.com. It cost 65.00 but comes with a 15.00 dollar rebate with 2.99 for shipping. It is a 80+ certified psu and is rated at 550 watts. It would handle a very decent card if he felt like upgrading in the future.
Any way, I would go with the 520 (based on the two choices), but if I could choose one I would go with this one.