SNT-TC-1A allows the employer to modify the requirements of the recommended practice to suit their needs. Provided the written practice defines the CWI as being the equivalent of the Level III as defined by SNT-TC-1A there is nothing illegal about the practice.

As is the case in many situations where a client is purchasing the services of a laboratory, the client has the obligation to review the laboratory's written practice to determine if the means of establishing and verifying the inspector's qualifications are in-line with their requirements. That is where many clients get something less than expected because they fail to read and understand the laboratory's written practice. It is a case of buyer beware.

Bear in mind that the CWI exams closely  replicates the examination process outlined in SNT-TC-1A, i.e., there are three parts similar to to those included in SNT-TC-1A for Levels I and II. A major difference is when the individual moves up to Level III and there is a recommendation the examinations include one part on the recommendations of SNT-TC-1A, the fundamentals of applicable NDT methods used by the employer, and discontinuities typical of different manufacturing processes.

Since the individual is already a Level III for other NDT methods, i.e., other than VT, I assume he/she may have already passed the "Basic Exam" described in the previous paragraph. The Level III only has to take the basic examination once, i.e., it does not have to be repeated for each NDT method for which Level III certification is sought. 

Whether this meets the client's expectations is a determination only they (the client) can make if SNT-TC-1A is the applicable document specified by the code or project specification.

Best regards - Al