Answer:
* Start with dear
* Say that you hope they get better
* Say that you, and their friends will be there to support them
* Tell them to not worry
* Tell them they can contact you if they need anything
* End of my saying sincerely, or love
Ex:
Dear (insert name here),
I would like to begin by saying that I'm so sorry for what you are going through. I really, really hope that you get better because I really don't like seeing you like this. It makes me sad that we won't hang out as usual while you are ill. I would like to say that all of your friends are wishing you better health and that soon enough you get better! I will always be here for you, so don't hesitate to tell me anything. And all of your friends will be here as well. I will miss you, and I know that you will get better soon, so no need to worry at all! Now be positive and don't be a pessimist. Sending so much love!
Sincerely,
(insert name here)
Hope this helped!