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_Chart_1618W.jpeg