I don't think prophetic dreams are very common, if they exist at all. I think what you see as prophetic dreams are probably just your unconscious mind pointing out things your conscious mind hasn't noticed. For example, with the dream where he was poking you, perhaps he had a slight change in his voice or his facial expression which your conscious mind, at the time, didn't think was important. Your unconscious mind, however, pays more attention to things like that, so it told you in your sleep that your friend's behavior is important.

I think the dream about your friend committing suicide is just your worrying that he will be gone from your life forever. I don't think you should take it as prophetic, otherwise it might be a self-fulfilling prophecy: if you think that your dream says your relationship must end, you won't make an effort to help your relationship, and then your relationship really will end. But it really won't be the dream predicting the future, it will be your actions that influence the future.