I find this question funny, because I think one of the main problems Kurt Cobain had was that he didn't really like being famous, and by default, probably didn't want to deal with people recognizing him and bothering him all the time.
So the answer is, I wouldn't say anything to Kurt Cobain.
Short Answer: I suppose if there was a situation where I knew I wasn't bothering him, and it was a casual forum like a tea/cocktail party, and he seemed like he was in a good mood, I might say, "Don't kill yourself?" but with a question-marky sound at the end. Then we'd both laugh, and he'd go kill himself anyway, because no one was going to be able to say the perfect thing to stop it.
Note: Nirvana rules. (The band, not the state of being. Thought that rules too. I know because I'm hella-enlightened.)