Well, there’s attorney client privilege, but it only counts if the attorney and client are working in that relationship. If Giuliani is running a foreign policy operation that’s not being the president’s legal counsel, that’s being a secret diplomat. Also attorney client privilege doesn’t apply if the two are committing a crime. Not clear whether that latter exception applies, but the first one does.
I would argue that it’s being a diplomat, and not a secret diplomat.
And then executive privilege may apply.
I find it rather remarkable that Adam Schiff can openly spy on people while investigating them for abuse of power.