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Firstly, its ridiculous to use the 5th in this case. The 5th is not supposed to be used because you don't "trust" the court (or congress) with truthful testimony and are afraid it may be twisted. The 5th ammendment protects citizens from having to testify to their own crimes, not from having to testify in an "unfriendly" atmosphere.
But the main point is that they should just simply grant her immunity and subpoena her. Straight up, no arguments, put her on the stand and find out what happened.