Clearly, this person was under arrest once he was handcuffed. From that moment on, any questioning was custodial interrogation
and, to the extent that such questioning occurred before he was Mirandized, his answers are subject to being suppressed in court
In the situation you described, your friend was not required to answer the very first (or any other) question. He did not have to tell the cops what he was doing; did not have to provide identification, did not have to get out of his car; did not have to consent to the search of his vehicle. However, once he did each of these things, the cops could legally look in the vehicle and the discovery of the contraband is admissible evidence against him.
What's wrong, primarily, was the fact that your friend, like most people, has a misconception of just what the cops can or cannot do and what response(s) are required of people when approached by cops (though, as I mentioned above, his un-Mirandized, post-arrest statements are subject to suppression).
Thanks again for asking your question here on JustAnswer. If you have any other questions about this situation, please let me know.