Listen, the best way to deal with this is going to be telling your mother that you know.

It doesn't matter what happens to that cop. He's probably cheated before, and he's going to cheat again.

The problem is that your mother is betraying your father.

And your father may be aware, or your mother is gaslighting him and lying to him and it's contributing to his depression.

You have to tell your dad.

The problem is already in your family. There's no avoiding that. Retaliating on the guy that your mother is cheating with isn't going to solve your problem.

But if you really want to cause that cop some headaches, I would get pictures and video proof that he's cheating, particularly if it's on the clock, he's in uniform, driving the police car, etc.

Next thing I would do is find some computers that are used by the public. Library, internet cafe if those are still around. You don't want to use any of your personal devices.

If your library has computers, they probably have computer passes. Use a computer pass, not your library card.

Make a throw away email on a public device. Who is in charge of his department? I would pick internal affairs. Send pictures, video, and screenshots of your proof to internal affairs. Do not sign your name or give any identifying information. Put something in the title of your email with that officers name.

And most importantly, NEVER NEVER check that email on one of your own devices. EVER. Only check that email on publicly used devices.

Assume that there are cameras in those places, so wear a mask, and wear something that will change your body shape, and not clearly recognizably yours. Ex: if you have a custom hoodie that nobody else has, don't wear that. Wear very generic looking things that lots of people own. Even better if it's something that you would never wear. If you have tattoos, cover them.

If you have long hair, tie it up and wear a hat. See where I'm going?