One who withdraws after encouraging or assisting in the commission of a crime may escape liability if he or she: renounces the other principals, turns the state evidence at the trial of the other principals, withdraws before the crime is complete

I believe the answer is: withdraws before the crime is complete.

By doing this, that person basically put his/her own position as a witness rather than the criminals.
But, the prosecutor could still has the ground to charge the person with the basis that he/she sure that a crime is about to happen but still does not report it to the police/