I recommend the approach from @MonkeyDoughnut with a bit of a twist.
Schedule an appointment with the professor to understand where he stands. If he is strident that BIL is a serial cheater and should be expelled and prosecuted, he should walk out, say “Thank you,” and contact an attorney that specializes in these things (they exist). If the prof seems reasonable, he can talk it through with him. See what his options are, etc.
The university holds all of the power here, so you have to proceed with appropriate caution. The punishments (if found guilty - sounds like that would happen) range from hand slap to point deductions to assignment failure to class failure to expulsion to revocation of degree.
If the professor seems reasonable, a simple question would be “If I knew this was wrong, would I have done something that was so obviously in violation of the rules?” The counter might be, “you signed off on the code of conduct, right?” And a response would be, “Yes. Just like we all read every line of the terms of service of every piece of software we’ve every purchased.”