How well do you know him?
After giving your commiserations about his health you could ask him how your friend is coping and if there is anything you can do to help either of them. He'll be worrying very much about his wife and might be glad to chat about that side of things.
If you have happy, shared memories you could say something reminded you of that time and reminisce.
If he has a particular skill or area of knowledge you could ask his advice - my fuchsia is being eaten by the slugs. Any tips?
That's assuming he was always a friendly, outgoing sort of person who enjoys a chat.
If he isn't, just say how sorry you are to hear his bad news and ask to speak to your friend.