The problem, as I understand it, is that the espionage laws - the ones that apply to the crime Snowden is accused of - specifically and deliberately _do not allow an exculpatory defence_.
Basically: You admit you drove over your neighbour in your truck? Sorry, you're going to jail - oh, he was about to shoot your cousin? Well, okay, that's a valid defence, we'll let a jury decide whether you went too far.
Snowden: You admit you told the public that the US spy agency that hired you was violating the constitution? You're going to jail - nope, don't care, you're going to jail.
The lawyer is in fact giving Snowden the best possible advice: stay out of the US, because the system is rigged. The laws were deliberately written to screw whistleblowers. Far as I can tell, at this point his options are basically a Presidential pardon (and Obama has stated he can't pardon Snowden before a trial, despite that being total bullshit; q.v. Nixon's pardon), staying out of the CIA's grasp for the rest of his life, or some kind of legal or political deus ex machina.