if my nephew with only a learners permit is driving his intoxicated father and gets stopped....the parent is being responsible by not driving, however what kind of trouble can he get in?

The law states that a the instruction permit entitles your nephew to drive a motor vehicle requiring a class D or G license on the public highways for twelve months when both of the following conditions are met:

1. The permittee has the permit in the permittee's immediate possession.

2. The permittee is accompanied by a person who has a class A, B, C or D license, who is at least twenty-one years of age and who occupies a seat beside the permittee.

I assume his father has a class A, B, C or D license.  The spirit of the law is that the licensed person is there to aid and observe the person learning to drive, therefore, being drunk is certainly going to impede that goal.  The law does not specifically state the licensed person has to be sober, however, the father and mother are liable for any injury to anyone else that may be caused due to an accident.  I believe that if an accident were to occur, and the nephew caused the accident with his father drunk next to him, they could be liable for a considerable amount of money.  I could also see a situation where an officer may arrest the father for public intoxication if the nephew is stopped for a traffic violation.  If that were to occur, the nephew could not drive the car without a licensed driver, and the car may be impounded.