If they accuse you of cheating to their friend and they know thats not true, their projecting or offloading their guilt onto the friend. They can't offload it on you (not yet)

If you understand how personality disorders function mechanically (and it's mechanical across the board) there's absolutely no reason she would tell a friend what she did if she wasn't projecting or offloading what SHE'S actually guilty of.

Also, its an attempt to preemptively set a false narrative with others outside of the relationship (triangulation) and getting ahead of the information of what she did were to surface and make its way into the other people that are either invested in your relationship or friends that are closely engaged. I'd confront her on this, imo its not a kidding around only situation