I think it's not possible. The first 2 numbers need to be the same in order to crossfire. If your buddy had lets say a 5750 it would be possible to crossfire.
An exception to this is the 5750/5770 can be crossfired with 6750/6770 because they are the same cards just under another name.
Also check this for future reference http://cdn.overclock.net/0/06/068d19bb_AMD_CrossfireX_C...
oh crap lol I was totally reading the chart wrong!!!! haha my bad for some reason I thought that the grey and white meant you could without the crossfire adapter (knew that was weird) damn lol my bad had an idiot moment