" . . . he knows an awful lot about stuff that would be very embarrassing to important people in the US administration (and in other countries). "

If you know that, he should have the right to remain silent and you should testify about it.

Besides, why should he testify that "I gassed civilians in my own country because powerful and unethical people in the American government gave me the capacity to do so"? To what issue would that be material?

-guest