Honor, it seems, is dead.
Why enter into a training contract, if one is unwilling to honor one's word?
If a pilot has left the employer because the employer has violated the contract or has not fulfilled it, then the issue is irrelevant: the employer has already broken the contract.
If the pilot has signed the contract and yet is unwilling to abide the agreement he made, the question is one of character, and ought not be how he can get out of it. This is always the question asked, though.
If the pilot has become unable to fly (loss of medical, etc) and thus cannot stay with the operator, training, or job, that's another matter entirely, and one should consult an attorney for guidance...rather than a public web board.