Agree. I've got someone lined up who has some experience giving second opinions in disputes. I was thinking "dear contractor, we don't feel you're competent to rectify this. See attached letter of support from [expert]. Pleas return [value] and we will be on our way" (obviously more verbose)
I'm not sure what letter you're going to write.
I would start with a second opinion from a well regarded professional. They may guide you on repairs, cost and course of action with the first fellow.
maybe then you can write a letter.