I have a question that I cannot seem to find the specific answer to in legal studies?  Can you please assist?

The U.S. Attorney contacts Mr. Plentibux, senior partner in a law firm, and tells him that a paralegal in the firm will be called to testify at a criminal trial against one of the firm’s clients. Although the U.S. Attorney does not know it, this paralegal has been working on a civil matter involving this client. Does this implicate the confidentiality rules? Does the U.S. Attorney have the authority to order this paralegal to testify? If not, who does? What would be the limits of the paralegal’s testimony, if she is ordered to testify? What do you think would be the limits of the testimony?