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that would negatively impact his client's legal position concerning past illegal acts allegedly commited for which counsel is retained to defend against. He could and would be presented with a choice to divulge only precisely that which could prevent the future commission of a crime by an existing client.
That Pres. Bush was not his client, as the DOJ is not the President's personal legal counsel in a particular criminal matter, what Mr. Tamm became privy to is not protected in any way from disclosure by him (to the best of my knowledge). Whichever lawyer allowed whatever information Mr. Tamm now possesses, if allowed to come into Tamm's hands inappropriately, would be on the hook for not properly safeguarding his client's (Bush or whomever) information/confidential secrets if disclosed to counsel with the expectation of confidentiality.
As far as statutory law, what's worse--breaking the law by not divulging a secret illegal warrantless spying program in direct contravention of the law or outing a known covert operative of the CIA for politically retributive purposes? Whistleblower protections are the weakest tea in the law, and do nothing to protect you from the political and economic retribution (much less the legal) that will necessarily follow.