Don't say that! Simply saying. "I'm so sorry" might be sufficient. When someone has a serious illness, perhaps even a terminal (deadly) illness, then I would keep the words to a minimum. Let them speak and respond accordingly. People who are dying or who are in the hospital for a serious illness will remember that you thought of them by doing things rather than saying things. Visiting them, perhaps sending flowers, these things mean so much to people when they are sick.