Even the sophisticated mitch4 is confused by this one: “So, what French/English translation oddity, or sound-alike, or pun, would make a plain American sympathy card look to a Frenchman like a thank-you card? I can’t figure it out. Or do I have that set.up wrong? “
Comics I Don't Understand 
Merci / Mercy ?
Or maybe something with “departed”. That could be part of a sympathy message, with a focus on the deceased (or “dearly departed”); while the French visitor, now leaving, can consider himself a departed. But I can’t craft a specific text for the card that would work with this idea.