It is best not to warn before a potential firing
People who think that they are going to be let go may say, "Well they are going to fire me anyways, what else are they going to do?" And then cause serious damage.
I'd be willing to be that not only is he not qualified for the job, but he knows that he can't do the job. Furthermore he's done things that indicate a serious lack of ethics already, so dropping below his current idea of "standards" is not where you want him to go.
Cheers,
Ben
I have come to believe that idealism without discipline is a quick road to disaster, while discipline without idealism is pointless. -- Aaron Ward (my brother)