Unfortunately, there's not much you can do about someone who is "planning" to do something like posting "revenge porn." He hasn't actually broken any laws or gotten himself in trouble. It's kind of you to be concerned, and your desire to prevent a disaster is understandable, but there really isn't an awful lot which can legally be done. Talk to a counselor about what you have heard, and hope that the counselor will bring this student in and warn him of the consequences of posting porn. The counselor will not reveal the source of the information, so you won't get into any trouble for making this "report." But don't be surprised if the porn gets posted anyway. Idiots are often truly determined to be foolish, and are not "scared off" by threats. And the girlfriend was a total fool to allow herself to be photographed so that there's permanent blackmail material. Put this into the hands of the counselor, and walk away from it. Getting more deeply involved than that won't help a bit.