I agree with @M_Barnes and @Warmheartsland. The Division Order department is who he needs to contact. Diamondback might have the wrong address and his money is sitting in suspense. They are pretty good about releasing funds quickly once they have what they need.
Here’s their contact information:
Diamondback Energy 515 Central Park Drive, Suite 500 Oklahoma City, OK 73105 Email: ownerrelations@diamondbackenergy.com Phone : tel:866-531-3667