I cannot help you with what the possible sentence could be, although all states usually punish Armed Robbery as a major, or capital offense. For example here in Michigan it is punished by a possible sentence of life or any term of years imprisonment. The state usually files charges against persons if they beleive that there is some evidence/facts which tend to show complicity in the crime. Your analysis regarding the other two occupants of the car is erroneous; being in the car in and of itself is not enough to show that one has acted as an aider and abettor in the crime, or as an accessory after the fact. To be an accessory after the fact there must be facts which show that a person knowing that someone has already committed a crime has provided some type of assistance to that person; for example, packing them food, washing or perhaps hiding their clothes, (again knowing that the person is "on the run"). Merely being present, even at the scene of the crime, is not in and of itself a crime.
As for the sentencing it depends on many factors which are too numerous to get into here. He should discuss this with his attorney. Beyond that, I see many questions on avvo in which people want to do what is going to happen to them because of such and such an offense or parole violation. In truth no one except your lawyer can even give you an approximate answer to that question because it depends on so many variables: your state’s laws and procedures, your criminal history if any, whether there are any sentencing enhancements, whether your state has diversion programs for your offense. These kinds of questions really require that you contact a local lawyer and even then they can only give you a range.