I get that you want to get back at your ex for cheating, but I don’t think involving your best friend in this situation is a good idea. From what you’ve said, it sounds like you’re still young, and decisions like this can sometimes lead to more drama or regret later on. Whether your ex cheated or not, giving him any sort of attention, even indirectly, might not be worth it. It’s usually better to just let go and focus on yourself and the people who truly care about you. Playing games rarely helps in the long run, and you deserve peace of mind. Do what feels right to you, but I’d suggest thinking about what will make you happiest in the long run, not just in the moment.
Also, you didn’t say your bsf is dating him just to get back at him. You made it sound like she betrayed you, that’s why I told you to drop both your bsf and ex.