I think for a Christmas card, especially since you have met him even if only once, it is nice to address it as Mr. and Mrs. as I'm sure others will say
I have friends I've never met the husband and we talk, text send birthday cards etc. but any Christmas or large holiday card I address to both or even if just to her if single and a child or young adult if living with one and she talks about throughout the year.
and yes, as @Goldengate8361,
said the less formal first names and last are nice too when friends and same ages especially.
"If you walk the footsteps of a stranger, you'll learn things you never knew. Can you sing with all the voices of the mountains? can you paint with all the colors of the wind?"