There's also the additional heat output, 2x power requirement, and scaling issues.
Just depends on what games he plays. Crysis, Bioshock, Mass effect, get the 5870 and crossfire later. Source games, Dragon Age: Origins, just get a single HD5850.
Im not suggesting that he crossfires right away, sometime in the future when games dont play very well on a single 5850
anyway 5850 FTW, the 5870 is no way in hell worth 100€ more than a 5870 imo
only if your rich i would consider the 5870 but with normal income i really suggest the 5850.