What does one do in a case of he said she said, when you are the one who has the truth but is the "liar"?

So heres the situation, i am trying to protect someone that is innocent. And another party is out to blame them for something that they did, only i cannot help the innocent party because i have been painted as the liar by the one who did what was wrong, and now the innocent person is getting ruthlessly hared and i just want to help so bad but i am powerless, and this person means a lot to me and i want to help because i cannot stand an injustice.