The GTX 460 would be an upgrade in power usage (GTX 460 uses less) and the gTX 460 is also a smaller card. As far as performance goes, the GTX 460 1GB is about on par with a GTX 285, so it would be an upgrade from the GTX 275. Also remember that you get DX11 capabilities with the GTX 460 too, and that's worth something.
Not sure why he wants to trade, but I'd do it.
