Let me unpack the story as I understand it:

A lawyer helps a client beat a murder rap believing her client to be innocent [back story or opening setup].
Only to discover new and compelling evidence that he actually he did the crime [inciting incident].
But because? of the Constitutional guarantee against double jeopardy, he can’t be retried [complication].
So she conspires to bring him “to justice” by a false charge of murder [plot].

Well, why not cut to the chase? She decides to murder him herself.? Get “street justice”.

The dramatic challenge for you as the writer is: how to make the audience at least empathize with her motives? Well,? I suggest that one way is the nature of his crime.? It can’t have been an impulsive crime of passion, done in a moment of anger (and under the influence of, say, alcohol).? Rather, his original crime has to have been premeditated and heinous,? say an act of prolonged painful torture on a helpless woman while she was alive, and depravity upon her body after she was dead.? ?In this way the narrative can sell? an audience on street justice emotionally? as justified even if logically they know otherwise. (There are any number of movies and crime novels that resolve the story with street justice.)

fwiw