If you were subponaed, you should talk to a lawyer about the possibility of incriminating yourself. Review the subpoena and see if it requires your presence. If it does, again, talk to a lawyer first and then consider just going in. No one said you have to volunteer all the information in the world and it sounds like you feel she did nothing wrong. So just be honest. You cannot be fired in retaliation for speaking because a) this is not your unemployment hearing and b) you didn't volunteer to be a witness, you were required to show up.