You've earned yourself a block.

aw ::c i hope you feel better
what did they do to you ::O

Please do not ask me again.

If your friend doesn't know about that "unspeakable", tell them.
If they do and think it's not enough to end the relations with said person, you could try to explain to your friend why this thing that was done to you is so hurtful/serious for you, if that doesn't help, then nothing really you can do, let them be. Or stop being friends with them both, if you can't stand it. Just don't try to sabotage or blackmail, it won't end well and will backfire eventually.
The "unspeakable" can be different things, so more specific advice is not possible, unfortunatlely.

(I'm just a passerby that stumbled upon this journal and decided to leave this input. Good luck.)

The "unspeakable" was that this other person rudely cut me out of their life, claiming to our mutual friend that I'm "toxic" and "harmful to their emotional wellbeing", which is preposterous to me because I have done everything within my power to be a good friend.
I think I will stop being friends with them both, though, because now that I think about it, our mutual friend is bad for me so maybe it would be best to remove them from my life posthaste.

Thank you for your input and well wishes.