Say the dog's name
A simple message is enough: "I am so sorry about Daisy. I know how much she was part of your mornings."
Offer one concrete help
Instead of "let me know if you need anything," offer something specific: dinner, a walk together, or help picking up food bowls when they are ready.
What to avoid
Avoid "at least," "you can get another dog," or anything that turns the loss into a lesson.