Answer:
Joe is facing an ethical dilemma
The main reason why it is an ethical dilemma is because Joe and Paul are close friends, and it would be hurtful for Joe to see Paul in jail, even if he has committed a crime, for years, and against his own company.
If the accountant had been a regular employee, with no personal ties to Joe, Joe would have obviously fired him, and sent him with the police. Joe would not care whether he went to jail or not, it would not matter to him. In this situation it does matter and he has to decide what is more important, his supposed friendship with Paul, or law and order and the financial situation of his company.