The other question I have is if a person commits a federal crime, does that mean he is definitely going to prison ?

Federal court is like quicksand. The harder you struggle to prove your innocence, the deeper they pull you. There is no way to predict the outcome of your person's case, but the feds have an unlimited amount of money and once they get their hooks in you, it is reasonable to predict a win in court - they win 97% of the time.