I think it's ~5% if they are at the same speed, and I think the 5870 can still reach slightly higher clocks. But certainly they are much closer when OC'd, I agree.
I'm just a fan of the waiting game if you already have such a good performing card (GTX 260). If he waits to when the 5870 is $300, he could either get that and have better performance for the same price for waiting, or get the 5850 which will probably be under $250 by that point.
Now I know you can play the waiting game indefinitely, but unless he's absolutely starved for frames in current games (Crysis, new Stalker, etc) why upgrade right now when Nvidia is 40-60 days away from finally launching new cards that may lower prices of ATi.