“Gute Besserung”

Word for word: “Good improvement”

/ˈɡuːtə ˈbɛsəʁʊŋ/

This is the standard response whenever someone tells you they're ill, from a colleague calling in sick to a friend in hospital, and it's often added at the end of a message or card. It works with both du and Sie, so you can say it to anyone.