You are correct that in order to be considered an Exempt Manager, they should spend more than 50% of their time performing Managerial work [1
This is a common complaint of Exempt Managers, and companies have lost a lot in Labor Lawsuits over this issue. The problem you will find is in correcting the issue in your husband's favor.
He can go to senior management and point this issue out, asking for either a change in their policies which promote high turnover to reduce the 70% to 50%, or he can ask to be considered Non-Exempt.
I think you can predict the possible response.
The other option is the legal approach. Typically, an employer can be liable for up to three years back OT pay if it is found they incorrectly classified an employee as 'Exempt' from the Fair Labor Standards Act.
Seek out a Labor Attorney for consultation.