I'm going to call out my ex on her BS in front of her friends
What behavior do you want to call out?
She's lied for no reason at all, denied lying, and stuck to her lies all along
You generally shouldn't call people out in public or in front of their friends for issues that only exist between the two of you.  However, if your ex's lies are affecting everything, you might be right to let them all know, and to try to get her to stop.
I can't keep letting her get away with it, but it might be too late to fix the relationship.
If that's the case, you should talk to her about it privately. I don't think there's a good reason for involving her friends. It would be better if you talked to her one-on-one.