My guy friend has been in love with his boyfriend since they were in secondary school, but since they got together, he’s been on a downward spiral. He used to be a smart little introvert and now he’s out taking class A drugs on the regs. The final straw was finding out he broke my friend’s arm during a drunk fight this week, my friend loves him too much to leave and I’ve always played friendly because I’d rather my friend be safe at my house than out fuck-knows-where but it’s too much now.
I need to try and break them up, permanently. I have a lot of personal dirt on the boyfriend from when he’s been coming over regularly and he’s told me stuff. None of it incriminating though, just shit childhood stuff. I’m willing to use it, but I don’t have an angle, and I can’t threaten him away because even though he is intimidated by me, he’d just tell my friend, and then avoid me.
I can’t tell my friends mum either because she will just “ground him” and he’ll run right back into the arms of his boyfriend… Help?
LemonVibe 
I get that you’re trying to be a good friend, but it is NOT your place to break them up, especially in such the manipulative way that you’re considering.
This is what I recommend you do instead:
Sit down with your friend and explain how you feel. Remind him of the person he used to be. Point out how much he’s changed since he’s been with his boyfriend, and how you see him going down a spiral. Explain that you love him, and you hate to see him like this.
And then … you leave it up to him to decide what to do. If you truly want to help, help him. But don’t try to do that by breaking up his relationship. That part is on him to do.
And definitely don’t trash-talk the boyfriend. If your friend is as much in love with this guy as you say, I can pretty much guarantee that the more you try to trash-talk him, the more you’ll end up alienating your friend, and the less your friend will be inclined to take your advice.