How can i write a letter to a friend who was recently diagnosed with cancer?

a close family friend (who’s still in university) just got diagnosed with cancer. i can confidently call her one of the strongest and genuine people i have ever met- i knew this even before i found out about her sickness.
i want to write a letter to her, not just for my condolences but also to make her stronger and happier on her journey. I’ve never known anyone in such a situation before, though, and don’t know what to say. please give me some tips.
sorry, didn’t know about the "condolences" part; this is all quite new to me. thanks for pointing that out!
ps: does anybody know what it feels like to go through chemo and radiation therapy? She will be going through it, and is very worried about it. i, however, don’t understand why, exactly- i only know it’s a horrible experience. any insight would be appreciated.