The only thing to do, in my opinion, is to go scorched fukking earth.
Tell the friend that if ANY part of him believes that you committed such a heinous act then you two are no longer friends and that this is your final conversation. Block him AND her on social media and refuse to have any contact whatsoever where a lawyer isn’t present.
I’m not sure if there is any progressive measures you can take to protect yourself in the event of her going to the police. Maybe take a recorded polygraph test? But i’d take all measures possible. Including cutting off any and all communication with ANYONE close to that woman or acquaintanced with her. If there is ANY legal proceedings that you can set in motion to keep her from spreading false stories I would look into that as well. Sue her. Set every nasty, freaky, sexual text she ever sent you on judicial front street. Any photos she sent you, videos, use it all. Ruin her credibility before she can even BUILD a narrative about you and your character.
Get a new set of friends. Forget them nikkas ever existed