The most helpful get-well messages are warm, undemanding, and specific. They let the person know they are missed, offer something practical, and do not ask for a reply.
That you are thinking of them.
Say it simply, and say what you miss about having them around.
Something to look forward to.
A walk, a coffee, their seat at the Thursday game, whenever they are ready.
A concrete offer.
A meal, a lift, watering the plants, covering a shift. Specific offers are easier to accept.
No pressure.
Tell them there is no need to write back.