Crysis 2 will certainly run at respectable settings on a single 6870 but you can't max it out with all the DX11 eyecandy unless you are running at 1280x1024 or lower. I'm guessing he wants Crossfire so he can max out anything he can throw at it. Most of the time the Crossfire will be overkill, but it might be advantageous to have it for a couple of very demanding titles.
A HD 6870 can max out metro 2033 at 1080p. And Crysis 2 better optimised than Metro and Crysis 1. So it means he can play Crysis 2 maxed out at 1080p with a single hd 6870.
I dont find any reason to CFX HD 6870. If he wants something better then he can get a HD 6950. It would be cheaper. And CFX 6870 he will need a costly CrossfireX ready board.