I can assure you Stephane that your idea, as much as I would like it to work, never will.
In this type of circumstance the ONLY possible route to avoiding being DFed is to immediately chuck out the BF, go home and throw herself on the mercy of the elders. Even that has a poor chance of success given her fade and moving in with a guy. They will most likely see through the façade, DF anyway and look to reinstate based on her subsequent actions.
A lawyers letter may delay or prevent an announcement but if she is not coming to meetings and living with a guy she is already spiritual toast to many of her so called friends.
If she wants to plead mercy with anyone then let that be her mother. Expend the time and energy reasoning with her so that she does not cooperate with the elders ("I really don't know my daughter's situation, why don't you talk to her, why have you not shown her any shepherding attention in six months, who has made this accusation, what proof do they have, why are they not following the scriptural process?) and understands that her daughter just wants to quietly get on with life.