Given that we're talking about (alleged) illegal behavior, I'd say that they were perfectly within bounds as long as a statute of limitations hasn't run out.
Otherwise, "retiring" becomes instant immunity from Congressional oversight. (Which is exactly what they're trying to do here, by the way.)
BTW, on a tangent, but related to Lerner, I’m all in favor of a limited exception to the 5th Amendment which would exclude the privileges against self-incrimination for government employees, officials, and elected officials with respect to actions made as part of their official duties.