I have a defense attorney that says that an Affidavit of Non-Prosecution is Brady per se?

Is it?

Depends too much on how one interprets the content and surrounding circumstances. Disclose it and decide later whether it is admissible for impeachment. That way, any decision involves a ruling of law on admissibility subject to appellate review rather than an ethical/constitutional conundrum subject to a writ and/or grievance.