Good evening. First, send the letter; then you can engage an attorney if necessary. But, in the letter, you want to raise the stakes on the contractor. Before sending the letter, I would get an estimate from another contractor for the cost to complete the job, including any remediation due to the substandard work done to date. Then, I would send the existing contractor a certified, return receipt requested letter detailing the history, terminate the contract and demand he pay what it will cost you to have the other contractor finish the job properly since you no longer have confidence in him. I would inform him that if he does not comply with your demand within a short specified period of time, you will have no choice but to file a suit for your damages. BUT, I would mention that if forced to file this suit, you will be filing this claim not only as a breach of contract case, but also as gross negligence, deceptive trade practice, and fraud causes of action, which will entitle your friend not only to his damages, but also an additional amount equal to multiple times his actual damages as punitive damages. That should provide plenty of incentive to comply with your demands; but, if it does not, file your suit on behalf of your friend. Even if you have to file the suit, that's likely all you will need do. In my experience, he will settle this without a hearing rather than risk punitive damages and the judgments being on the record.
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