Putting on my lawyer hat for a moment...
1) Generally it is not a crime to
NOT report the crime of another, although there are exceptions (as with an ongoing criminal conspiracy, which may or may not be pertinent here), in which case suppressing the information (or lying to authorities about it) may constitute being an accessory;
2) If someone has been pardoned, they
cannot "take the fifth" with regard to their activities
for which they have been pardoned, since they cannot "incriminate themselves" with regard to that testimony. If, however, they lie (perjure) under oath, they can be charged with that crime, and they can be placed in contempt of court (or Congress) if they refuse (like
Susan McDougal). This is somewhat different than immunity, which can be broader in application. Often someone who has been pardoned will be granted immunity for other uncharged misconduct so that they
can be compelled to testify.
