I agree. From the photos it appears that all of the mass footings (i hate using that term with such lousy construction) were installed too shallow and have heaved. Somehow, 6 of the 7 (assuming a hexagon gazebo) have heaved a nearly equal amount.
You really need to fully remove and reconstruct all of the footings, excavating them to below the frost line. Is there a way to dismount the gazebo and set it to the side on blocks while the footings are being reconstructed? Using two rough terrain hi-los perhaps?
He could fix it from the outside by adding 2 beams and a header between the beams on each side of the center . Then he would not need the center footing.