My sister had leukemia and sadly died of it, but make sure that you pray for that person. Make sure that you tell them how much they mean to you. You also need to be with that person by visiting them frequently so they know you will be there for them.
You pray for them at first. Try to know their sickness history and if you can do some researches on how they might get well. If the result were bad then just be with him/her and enjoy every second with them and make them feel really happy and glad you are there.