Just like fraud in any other contractual agreement. He said he'd not do X and then did X which resulted in his unjust enrichment and/or extra expense to the oth

Just like fraud in any other contractual agreement. He said he'd not do X and then did X which resulted in his unjust enrichment and/or extra expense to the other side. Not saying it happened, but "you should have/could have stopped me from violating the contract" is not a defense to fraud.