Good point, Sam. If Miller isn't the target of the investigation, then her 5th Amendment rights are meaningless. She can assert them on the stand, and the prosecutor could force her to testify by granting her immunity from prosecution.
She wouldn't have to accept immunity.
I could be wrong, but I think the 5th can be stretched to cover testimony that you would give not only to directly incriminate you, but that could allow the prosecution to develop a chain of evidence that leads back to you - for a different charge.
but since she didn't take the 5th, I guess this doesn't apply anyway.