Not so sure. The rules are different than normal oversight testimony.
She can take the fifth of course, and be given immunity and forced.
Also. Joe’s controllers may have decided that he has become a liability. Dangerous thinking because Joe, or more likely Jill, can bring it all down in a fit.
They really don’t have to worry about taking the 5th because as a prosecutor performing work activities she would have immunity for just about anything she did on the case.
It would have to be egregious for any coverup to be considered criminal or even illegal.
This is one of the fundamental problems we have with the US legal system. Immunity for government agents, prosecutors and judges puts these people above the law and incentivizes them to cheat, protect themselves and their allies and violate due process rights of political enemies when they are defendants.