Thank you for sharing.
That verse is actually very timely. It's the biblical instructions for shillspotting!
In this context, it looks like she is trying to tell him that just because his mother told him it was not possible that he had done such a thing doesn't make it the truth.
See, divisions and obstacles look like this.
"Of course that didn't happen, this is silly talk!'
"You have an overactive imagination"
"People misremember abuse all the time"
"As your (parent, guardian, handler) don't you think I know better than you what you did and didn't do?"
Smooth talk and flattery is a common tool for manipulation.
And of course, being part of a cult means they create your reality for you, and all other reality is a lie (so they say).
If you suggest something that goes against the created reality, well of course this must be nipped in the bud, and it is done under the guise of - you got it - smooth talk & flattery.
In the story you told - they were both children. Who were products of their environment and didn't know any better. Very likely mind controlled.
A way I could see to at least begin to fix and heal this, would be for him to sincerely acknowledge and apologize, since he is out from under the mind control and knows better now.
Taking responsibility and making amends for any harm you did to another is one of the steps to maturity and yes, even spiritual enlightenment. At the very least, it makes one a better human being.
I hope this helps, and if you wish to go more in depth, feel free to expand the story :)
Wishing you, your ex, the girl and everyone else involved freedom, healing and peace.