Jeremiah, your friend's experience is not all that uncommon.   I often get a few residents thanking me for introducing a few less-familiar songs when I do. ("Ripple," for example, usually gets a very positive reaction, as does "Proud Mary," even though most residents are not very familiar with those.   In St Pat's programs I usually do "Star Of The Country Down" with good result.   Yanks are usually not familiar with it.)

Still, a lot of residents do respond best to songs that they are somewhat familiar with, even if they would prefer that not all entertainers do the same songs.   (You Are My Sunshine is still probably the most requested song in all my senior gigs, though.)   

I would add that if you play instrumentals, you can keep the audience interested and involved with a much wider array of music, familiar or not. Especially if it's upbeat and catchy.
If you sing in a language the residents don't understand, or if you sing lyrics they don't know, you may lose a lot of them.   But if you sing no lyrics at all, they seem to care a lot less whether they know the song or not.