There's even some grounds for discussing addiction in an Iron-Man/Avengers promotional interview with RDJ, in that Iron-Man 2 (and to a lesser extent the other films) adapted some aspects of the "Demon in a Bottle" storyline from the comics to show Tony's alcoholism, though never to the depths that the comics did.
I'd say in that context it would be appropriate to bring the subject up, though again, the reporter's mistake (or rather, deliberate choice) in this case is to think that his role is to be adversarial and ambush RDJ rather than to simply ask him beforehand if that would be okay to discuss and let him decide. There's a time and a place for ambush tactics and investigative journalism, and that's when you're exposing corruption or crime, not when you're interviewing movie stars about their latest projects.
Now see, that's what an intelligent reporter should and would have done.