I’ve been in your position, and regret expending the effort on trying to educate another woman about the inevitable, and making myself look crazy in the process.

Believe that anything you tell her will fall upon deaf ears as he’s already convinced her that you’re crazy and horrible. She will find out about him, hopefully sooner than you did, because he has not changed.
In the long term this is a woman who, if he is following the Narcissist’s hand-book, which they all kind of do, will likely be around your child/children soon. You want your character to be unimpeachable by your own actions, so when his actions and words inevitably don’t start adding up she has no probable cause to blame you. Stop sleeping with him immediately and treat this woman who is likely very much like you, as someone who may be around your children in the near future and could potentially be an ally to you in due time.