If he finished his probation, there is no problem. If on the other hand, he still on probation, and gets a dui while on probation, the state may file a motion to revoke the initial probation.
Unless a warrant for probation violation is taken out and the current non-reporting probation sentence is stayed, the State would not be able to retroactively revoke his probation following a DUI plea. However, if he is still on probation, even non-reporting probation, when he enters a guilty plea to the new DUI, then the balance of that probation could be revoked. In fact, the State would not technically have to wait for him to enter a plea to the DUI charge -- if probation learns of the new arrest, they could file a petition to revoke his probation even while the new charge is pending. He would, of course, be entitled to a hearing, and the State would have to prove that he committed a new offense, but it is possible for him to face a revocation of the current probation. He should seek representation by an experienced criminal defense attorney immediately.
If you are on non-reporting probation, that is still active probation. A DUI arrest is a violation and the state could move to revoke. Also, there is an obligation to report those new arrests while on probation. The revocation would be limited to any time that you have left on probation unless it was under a First Offender Sentece whereby the court could resentence on the felony and revoke you on the balance. There cannot however be a retroactive revocation.