If he goes with a E5200 system, then his only upgrade would be current s775 cpu's. Also the E5200 is expensive at $96. The 7750 is only $63.
If he goes with the 7750, he can upgrade to a Phenom 2 later on. If you overclock the 7750 to 3ghz, which will be very very easy, you'll be fine with your 4870, especially if your running at higher resolutions like 1680x1050 or higher.
Id recommend this over the 7750 though:http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.165141
$123 after $10 rebate. (Dont forget a cooling fan)
The best scenario would be for your friend to save for a i7 or P2 AM3 system.