This argument falls apart when you extend it a little bit further.

Suppose that same public defender got his client off, and in their last conversation his client tells him "thanks! I'm going to go rob a bank." Now that public defender would be both legally and ethically obligated to report said crime.

Why? Because the default state is this - if you know of a crime, you are obligated to report it. The public defender example is the exception we carved out because we as a society believe it's required for a fair trial. Not so for money laundering.