well i bought a 5850 and is EXTREMELY happy with it, the 5870 isnt worth a 100 euro more. and if you want you can overclock the 5850 to be as good or better than a 5870
i bought the xfx 5850 xxx edition and have overclocked it to 950/1250 @ 1.16 volts and it runs very cool at those settings, in furmark it goes up to about 70c. i get a little over 17000 gpu score in 3dmark vantage
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.