Not enough information has been provided by the original poster to reasonably discuss this.
At its root, this is a legal issue and is best addressed through an attorney.
As an ethical issue, inadequate information is presented.
It appears from the description that the subject son has departed the employer, although this is far from clear, and if so, then the issue of whether he should leave is irrelevant and past tense.
In legal matters, the devil is always in the details, and we have no information regarding the specifics of the contract, nor of the circumstances of separation, nor of the nature of the legal violations.
It's certainly possible that a demand by an employer to act illegally will figure into the duty to abide a contract, but that depends both on the demands placed on the individual by the company, and also on the language of the contract, to say nothing of the actions of both parties. For example, if the company made an assignment that was clearly illegal and the subject accepted it, vs. refused it, relative to terms that establish whether the subject is fired or let go for cause, or self-terminated or resigned; the language of the contract and the way it's adjudicated may vary considerably.
One contract may stipulate that if the employee separates for any reason, the balance of a pro-rated amount is immediately due, or even that the entire balance, unprorated, is immediately due. Another may stipulate termination for cause, or may stipulate that if the employee elects to separate, the agreed amount is due.
Extenuating circumstances may apply; the contract may or may not be mitigated, and there's insufficient data to address that. It's a legal issue; an attorney consultation is in order. There are no protections provided by a public airing of these matters, that would otherwise be provided in a paid consultation in which the attorney-client relationship is formally established.
A general discussion might be had about breaking a contract, but for any meaningful discourse, some context needs to be provided. It's not legal to keep a 135 employee on a 24 hour leash, but it still goes on with tacit approval by POI's throughout the system and is not unknown in the 121 world either (certain ACMI, for example). This method is not legal, but one may have a harder time building a case around what amounts of a common standard. If, on the other hand, the subject pilot was told to fly an unairworthy aircraft under penalty of termination, or told to take revenue legs after being legally unable to do duty for the company, the matter may be different.
Under 135, I think at this stage I've seen most of what an employer could possibly do, from pressure to take flights ferrying drugs in the night to fly airplanes with cracked wings. I've had offers of cash under the table to take illegal charters, been left in the middle of nowhere in the winter with no clothes, shelter, food, or money, and told to replace both engines by hand, before I could leave. I've been handed aircraft with cracked gear, and I've been told point blank in weather conditions far below minimum, when refusing a trip, that the DO didn't care if I survived or made it back, but that I must depart to show dispatch reliability. I've had suicidal individuals put in the cockpit with me twice, and once an indvidual with a heart condition who collapsed while I was flying...who'd been put there to save the cost of a nurse. I've had a man pull a pistol in flight behind me and I've been ordered to fly him, and just about everything else one can imagine happening. I've had an employer pick me up by the throat, as a kid, and threaten to put a shotgun in my mouth and blow my head off if I didn't comply. There's very, very little one can relate about what might happen in this industry that will surprise me.
That said, there's a lot I hear from new pilots in which their sensibilities are offended, and my response will be to man up, focus, and look at the larger picture for a moment. Pick one's battles might be the best way to address that. Again, details matter. If it's egregious and unsafe, then one shouldn't do it. As a general rule, every flight must be legal and every flight must be safe; compromising either one is grounds for not making the flight, and I've refused a lot of flights as a result. That said, see answer A, and pick one's battles. Judgement is often knowing, and judgement comes from doing. Part of doing is not doing...refusing when one must. Inevitably, one may find one's self refusing that which one ought not, but the limits must be learned, and in some cases, those limits are somewhat of a sliding scale. In our business, experience is largely about understanding how to negotiate the grey, and there is substantial.