Prosecute the judge as an accessory after the fact.
He is helping the guy escape justice. That is a crime.
He's not "helping the guy escape justice," he just has a different view of justice than you do. Unless there is some evidence of bribery or other corruption, impeachment (or possibly recall) is the only available option, and even that might be a stretch, unless he violated some law regarding minimum sentencing.
I'm not sure what the process is for removing a county judge in Montana (or if he's appointed or elected), but I doubt disagreement over sentencing is valid reason for impeachment (although it might suffice if he's subject to recall.) A previous poster says he's about to retire; if that's true, I doubt anyone will expend the energy to remove him.