I think my ex-husband's new girlfriend is cheating on him.
What makes you think she's been cheating?
I heard from my friend that they have been cheating on him.
If you have any goodwill towards your ex, you should tell them about that.  People deserve to know when they're being cheated on.  I'm sure if the tables were turned and you were being cheated on, you'd want to know, right?
Yes, but I have no love for him and I want to see him hurt.
It's right to tell the truth. Imagine if the roles were reversed, you'd want your ex to tell you, right?