Say "hm, not what i expected." then i'd keep eating. As much as i do like cats as pets, the idea of eating one does not freak me out unless it was a pet i was familiar with. I WOULD ask where they got cat meat from, since i don't think the local deli has that in stock. Maybe even get the recipe. Seriously, if it tastes good then what is the problem? I DO draw the line at human flesh though, i don't care how good it is, i'd spit it out and hope i am not next.
With exception to the vegetarians (who wouldn't have eaten the meat anyway and probably not have asked what it was), all the meat eaters here who would wig out and curse the friend and his/her family and never return, are pansies AND bad friends.