The range of outcomes here runs from a stern warning all the way back to finishing the original sentence, and where your boyfriend lands depends on several factors that will become clearer after the magistrate hearing.
Driving without a court-ordered ignition interlock device is not a minor technical slip. The interlock was a condition imposed by a judge as part of a sentence or probation agreement. Violating that condition tells the court that its orders were ignored, and judges do not respond well to that, regardless of how minor the underlying act seems.
The magistrate this morning will likely set a bond or hold him while the violation gets referred back to the original sentencing judge. That referral is where the real decision gets made. The sentencing judge is the one with authority to determine consequences, and that judge is working from the position that leniency was already extended once and was not honored.
Best case is a slap on the wrist, a warning, stricter conditions, and release. Worst case is being sent back to serve part or all of the remaining time on the original sentence. Courts are comfortable letting someone sit while the violation process plays out, and there is no urgency on their end to resolve it quickly.
What works in his favor is a clean record since the original sentence, a credible explanation for what happened, and an attorney making the best possible argument before the sentencing judge. Without representation, he is walking into that hearing at a significant disadvantage. Get a lawyer involved today if at all possible.