musiclover, how did you discover the truth? Did it come straight from the doctors? If not, perhaps it is someone else's interpretation and as such, not as reliable as information directly from the doctor.
If the doctor did indeed speak to you about your friend's condition, maybe you'd feel comfortable enough to ask *why* your friend has not been told the extent of his illness. The request may have come directly from your friend (i.e. "I don't want to hear what's wrong - tell me how to fight this!")
What a difficult call to make. I'm sorry you've been put into that situation. If I were you, I think I'd do whatever I could to verify that the information you got is accurate and realistic before making a decision about talking to your friend.
Blessings and best wishes during this difficult time,