Old Man is Sinning, Not the New Creation?

Tim refutes the idea that some come away with from 1 John 5:18. The error that born again believers have two natures: the old, and the new nature, the old and the new man. So, they say when a Christian sins, it is the old man who is sinning, and not the new creation, because the new creation cannot sin.

Original Question: Together with a friend, we were thinking about 1 John 5:18. About the christian who cannot sin (keep on sinning). Now my friend explained it like this: Christians are born again, they have two natures: the old, and the new nature, the old and the new man. So, when a christian sins, it is the old man who is sinning, not the new creation. Because the new creation cannot sin. So, even when a “christian” is sinning, 1 John 5:18 is still true, because it’s not the person who is sinning, but only the old man is sinning. I believe the text isn’t telling us that. The danger of this view is we can nullify all warnings in the Bible, because we (our new nature) can not sin. Is this view wrong? How wrong and how to refute it?