I'm not certain, but I don't think that there's anything in American law that requires that we report (without being asked) any illegal activity that we may be aware of. Other than a moral requirement, that is. But not a legal one

But if asked by the police, it can be considered obstruction of justice, a criminal offense, if we lie.

However, if called upon to testify in a court of law, then naturally we have a legal obligation to tell the truth.

An American cannot, however, be forced to testify against their husband or wife.