I don't know what to write...?

Tell her how much you have appreciated her support of the soccer team, and how much you enjoy your friendship with her daughter. The closest I suggest you get to the health issue is to say that you're very sorry to hear she isn't feeling well and that you will be thinking of her. And that is it -- everything will NOT be OK, and it's really not appropriate to say that it will. And stay away from religious stuff -- that is very personal, and will be a matter for her very close friends and relatives. Oh -- and sign your full name and put your name and address on the envelope!