They're not actually investigating her, they're investigating a source that leaked her classified materials.
If they're not investigating her she doesn't have any 5th-amendment protection and can be compelled to testify on anything relevant, including how to unlock her devices.
Did the individual store the classified material in the bathroom at his beach-side resort?
If they're not investigating her she doesn't have any 5th-amendment protection and can be compelled to testify on anything relevant, including how to unlock her devices.