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.http://www.newegg.com/Product/Product.aspx?Item=N82E16814127585
It has a lot of good features and would do very well in a budget system like your friend's.
It costs about 75.00 dollars plus 7.00 shipping.
hope this helps