Is there some rule that says he can’t testify after he has resigned?
None that I know of or no more so than if he were still with the administration. What it does is send a message to anyone else who would ‘rat’ out this administration. Remember what Jarrett said to her staffers in Chicago before the election even took place. It was to the effect that those who betrayed the One would have hell to pay. As the story goes, they all signaled their approval with the statement.
The timing of this makes me suspicious. What is the time line of this affair? Is it recent or has it been going on for some time.. How was it discovered.. I don’t trust this group..