I am glad I won’t be driving that BMW. The work described constitutes a “repair”, not a “rebuild”. A rebuild would restore the caliper to “as new” condition - a level clearly not achieved. Two critical mistakes were made in that repair. First, all that rust means moisture was present between the dust boot (outer seal) and the hydraulic seal (inner seal). No mention was made of cleaning the rust from this area. Any residual rust here - especially with all that pitting on the piston - will eventually re-seize the piston (more likely sooner than later). Second, that piston is junk and must be replaced. All that residual rust and pitting will prevent the seal from seating and will eventually destroy the all important hydraulic seal. Remember we are trusting our lives on high pressure hydraulics. Even small imperfections will result in leaks. Fortunately pistons are not expensive. Unfortunately they are not included in a “rebuild” kit. A third consideration, particularly in a high-end car, is: was the bore chrome lined? If the bore was lined all that rusting indicates the chrome is gone from the area around the hydraulic seal. Any rough edge of the remaining chrome can damage the hydraulic seal. It will take a qualified professional shop to restore a chrome lined bore to “as new”.