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If he can get a Public Defender to assist him and if the PO feels it is going to be more helpful, they can release him to the rehab before the 120 days.
Though the thought behind the 120 day is that the offender will get clean and so they also could force the 120 days and then go with a rehab. Unfortunately, it is tough to be definitive, because each case is different and weighed and evaluated on a case by case basis.