How would you [or how do you] address this situation?
I think you have to treat the situation as you would want to be treated yourself.
If you truly believe someone to be absolutely unsafe, having discussed it quietly with colleagues if possible, then you have to pass your thoughts on to a relevant person, in the knowledge that they are likely to be at the least investigated, or suspended. You should be prepared to stand in front of them and justify what you have said and done.
If you are not sure enough to do that then, in my opinion, you're not sure enough to act.
If you're sure, but not prepared to say something and justify yourself for saying it, you need to look at your multi-crew communication skills!
Lafite