When you don't know the spouse's name

[TootsNYC]

You could have called Mr. John Smith and asked for his wife's name to write into your address book. Or called someone who also knows him, and asked them what her name was because you just had a mental blank.

And sending a separate card could have been softened by adding a note--"Julie--I'm late to hear the news that you and John reconciled. I'm glad for you, and I wish you both the best."

[bopper]

In that situation I would address the card to "The Smith's" and inside the card would be to "Becky and family".