Hmmm. A very knowledgeable emailer says it's a felony ...

Invoking a privilege is one thing, but telling a person not to show up in response to a subpoena -- if only to actually invoke the privilege -- is quite another. It's not just worse, it's a felony under federal criminal law. See for yourself.

18 U.S.C. Sec. 1505 : ... Whoever corruptly ... influences, obstructs, or impedes ... the due and proper exercise of the power of inquiry under which any inquiry or investigation is being had by either House, or any committee of either House or any joint committee of the Congress ... [s]hall be fined under this title, [or] imprisoned not more than 5 years ... or both.

18 U.S.C. Sec. 1515(b): As used in section 1505, the term "corruptly" means acting with an improper purpose, personally or by influencing another, including ... withholding, [or] concealing ... information.

I can't vouch for the legal interpretation. But it seems like decent prima facie case and the emailer knows his stuff.