In arguing, there are two types of arguments - deductive and inductive. With a deductive argument, the conclusion necessarily flows from the premises. In an inductive argument, the conclusion is probable given the premises.
For example, I might say, "I think George Bush exists, because I've seen him on the news, I've heard the testimony of other media sources that he exists. Therefore, he exists". This is an inductive argument which does not guarantee the truth of the proposition "George Bush exists", but rather makes it highly probable it is true. Other premises added to the argument might make that probability lower.
To call this argument by Plantinga "The Chance Argument" that you've heard 10,000 times before is misleading. This is no more of a chance argument than any other inductive inference, and so shouldn't be accorded special status. It needs to be considered on the basis of the probability of its conclusion. I agree with Plantinga that the probability of R given N&E&C is low or inscrutable. Your task, if you think he is wrong, is not merely to call this "The Chance Argument". You need to provide additional premises that are not question begging, to show why he is wrong. Perhaps you want to show him that his argument doesn't have an undefeated defeater for naturalism? Perhaps you want to show that in fact R is highly probable. As his argument stands, though, the probability of R seems low.