Although based on the little that has been said it seems that the nephew would be charged with assisting or aiding in the theft if he were in the car when the theft was committed, it is possible that the prosecutor has good reason to believe the nephew had no prior knowledge of the crime or the prosecutor believes he will be unable to show the nephew had the criminal intent necessary for a conviction.
Whatever the case, it would be smart for the nephew to have the notice he received from the court personally reviewed by an attorney. He will want an attorney before speaking of the theft to others (certainly before speaking to the police) and he will want an attorney if he is, in fact, being charged with "acting with another" to commit theft.
But a quick call to the court could explain the letter and what exactly is required of him to see if he needs an attorney representing him or if he will only want one to help with telephonic testimony.
The nephew just needs to make sure he does not miss any court hearing by failing to respond properly and in a timely fashion to the letter. A court summons is not something he should ignore.