Thank you for your question.
Your tax information is protected by your constitutional rights to privacy unless that right is abrogated by contract
(i.e., you give someone permission to see it), or if you put your income at issue in an adversarial legal proceeding.
Under a disability policy such as the one you are referring to, the policy requires that you cooperate with them in connection with any claim submitted. Further, because the nature of the relationship between insured and insurer is one of contract, the company is not in an adversarial position to your husband that would trigger a constitutional privacy protection. In other words, the insurance policy provides the insurance company with permission to look into this type of information.
The type of information that the insurance company is requesting is related to the nature of the disability claim. A disability claim is not only associated with the actual medical disability suffered by your husband, but also concerns the calculation of benefits due to him which is based in part on his prior income. Thus, they likely have a legitimate interest in obtaining the information. Also, you do not want to endanger an assertion by the insurance company that you have failed to cooperate, which will trigger their rights to terminate the benefits.
That being said, the insurance company is definitely undertaking a review of your husband's claim. They are looking for any information that will allow them to reduce the benefits. Unfortunately, they are most likely entitled to this information pursuant to the insurance policy. If there is any information out there that would trigger a reduction, such as a miscalculation of his earnings, then the insurance company is entitled through the policy to be able to review the information to make a determination.
You will need to comply with the waiver request. If the insurance company decides to change your benefits based on any of the information provided, then you will want to contact a disability insurance attorney to protest the negative finding on your claim. However, at this point, you are required to comply with the information request.