If the prosecutor wanted to charge Frank with embezzlement, then Frank would already have been indicted.
Conclusion
Frank is not an embezzler
Evidence
If the prosecutor wanted to charge Frank with embezzlement, then Frank would have been indicted. But Frank has not been indicted.
Evaluate
The proper conclusion we can infer from pairing that conditional with the fact that prosecutor didn't indict Frank is that the prosecutor didn't want to indict Frank.
The author's argument is making an illicit leap from "the prosecutor didn't want to indict him" to "he isn't guilty of embezzlement".
Maybe Frank is an old friend of that prosecutor, or Frank works for a mafia kingpin who owns the prosecutor.
Goal
We want to see some sort of conditional premise: If A wanted to do X, then Y.
Then we want a factual premise that triggers the contrapositive. But not Y.
Then we want a conclusion that goes too far. Instead of saying "A doesn't want to do X", it makes a speculative leap about what that means, even though there could be other possible explanations / interpretations.