I think it all depends. How close are you to this friend? Does the friend know? I mean, is he cognizant of the condition and aware enough at this point?
If he is, just talk to him as you would normally - whatever that was *before* you found out. You can certainly tell him that you're sorry, but I would wait to see how he's taking it before trying to find words just to say TO him. I think YOU are trying to convery YOUR feelings to HIM. That's normal, but maybe you shouldn't burden him with that, yet. (except to show you care, and can be there for him, if you can, etc.) Ask him how HE feels about it, ask him about his treatments, talk to him about the knowledge he has about it now and how he thinks it might affect his future.