Great question, I don't know, that would take a few JAG officers I suppose.
To be clear, Petraeus retired from the military in summer 2011.
Ping.
I though he was still active, too. Musta missed that one, Haden served while still ADAF. Not necessarily a bar to testimony to have an IG investigation ongoing, but they’d have to recall him to AD in order to give him an order not to testify - and even that might not be a “lawful order”.