People who testify to grand juries are automatically granted immunity with respect to that testimony, unless it is explicitly waived. That automatic immunity negates their right to invoke the 5th amendment under the law.
Wild guess: she's asked to testify about Assange. Assuming he's guilty, either she incriminates him or lies. Lying to the jury wouldn't be covered by immunity, would it?
No, lying would not be covered.
Nothing to gain, everything to lose. She's doing the right thing. Huge self sacrifice.