I learned this many years ago,....when you get a request and don't know it ALWAYS have an alternative song for them be very diplomatic, for example:

"Excuse me can you play Moonlight Serenade for us please"?...

your answer could be, ....

"Geeze I'm sorry, it's been so long since I've played that one but how about "Unforgettable"? and he say's ok swell,
that's a great one too thank you...

Now the husband goes back to the table, tells the wife honey he didn't have "Moonlight Serenade" but he's gonna do "Unforgettable" for us ok? you gracefully get out of the unknown request predicament,.....He's happy, she's happy, and they dance and hold each other, ....and best part they love you for playing a song for them !!