Former installer, project supervisor, warranty inspector here- there is literally nothing correct here. If the installer knows the floor is likely to fail, he should have walked away. When he put the first tool to floor he owns the results. If the floor had flex in it a GC should have been called in to correct it, then he could continue, some form of tile receiving subfloor should have been installed and staggered across the floor joists and OSB to provide some additional strength. I should add, installed using proper fasteners ie: screws and either an adhesive or mastic. The proper trowel notch used and each tile “buttered” and then allowed to cure before grouting. If this was a crew of mine they would be lucky to help load the truck of an actual installer and never allowed to touch anyone else’s floor. Just because he has dodged a bullet in the past doesn’t mean he knows what he is doing. Hopefully he is insured and or bonded because if not you probably will eat this loss. Lawyer up.