I assume the agent has looked into an NFIP policy (and maybe private policies too) and this is the result of the property value being deemed below the NFIP deductible? Is that accurate? If so, it would not be insurable because the owner would be purchasing something that would never be of benefit them. If the value of the property is not below the NFIP deductible it seems demolition value would be one option to consider in determining the insurable value. You want to ensure adequate coverage but not more coverage than the customer will ever receive the benefit of.
Look at Q&A #9 on p. 8: https://www.fema.gov/media-library-data/20130726-1742-25045-5644/interagency_q_as.pdf. The proposed q&a #9 from 2009 specifically referenced demolition as a method to determine value. The finalized q&A linked here from 2011 doesn’t specifically mention it; rather, they leave it open to various methods and do not exclude demolition as an option.