She does not have to talk though.
And she cannot be forced to either.
She could if she wanted, but she would just be an additional witness to the already mountain-high pile of evidence allegedly gathered by the FBI.
Also, if the "co-conspirators" get their day in court, and if these trials take place before a jury, Ghislaine's testimony could be seen and treated with truckload of salt. Because at this point she is a convicted felon rating on her co-conspirators; her doing so could be viewed as a way for her to cut a deal.