I feel Michael Douglas over stepped his bounds as far as publicly revealing Val Kilmer's battle with oral cancer. It is apparent Val Kilmer did not want his medical condition made public, and Michael Douglas--while being a close friend--should not have taken it into his own hands to announce it.
Yes, I believe Michael Douglas should apologise to his friend for revealing confidential information. Although concerned about his friend's reluctance to discuss it, Douglas should still have consulted with his friend in the first instance before making it public. It is also possible that exposing such a private matter may make it harder for Douglas' friend to mentally cope with his illness.
Yes, Michael Douglas should apologize to his friend for revealing private medical information before he was ready to talk about it. There is no reason a friend should tell others about a person's illness, especially if the one who's sick does not want others to know. It crossed a line.
Health issues are highly private to each person. How much information anybody wishes to divulge to others is totally up to them. If Michael Douglas wanted to go around talking about Val Kilmer's health problems he should have cleared it with Val first that he was happy for him to discuss it publicly. He didn't and so now he should apologize.