I used to work for a company that did model risk management consulting for large banks and source code reviews were a standard part of what we did. What sounds different from the OPs situation is that it is the customer who would be conducting the review and not a third party. Take everything you read here with a grain of salt but it would be best to consult a lawyer. Even if you hold the patents for what your software is doing under the hood it may difficult and expensive to sue in the event that your customer does simply copy your secret sauce