Treading lightly...

Hello, all.
In writing our thank you cards, we've run across a bit of a conundrum. One of my dad's friends came to the ceremony, but left soon after. He gave us a card and monetary gift. However, he signed the card from himself and his late wife, who passed suddenly from a heart attack after which he found out she kept heart problems from him.

My question is, how am I to address the thank you card? I don't want to be callous as he is a good friend of my dad's. I'm not sure if she already had the card ready to go when she found out about the engagement, or if he is experiencing complicated grieving secondary to the suddenness of her death, or signed the card that way out of habit. Either way, I'm confused about how to address this thank you card and would really appreciate any advice. Thank you in advance! Brittany