Two fold problem. The paint thinner is a solvent, which if anything made the situation worse, by forcing the glue residue deeper into the concrete (similar to oil stains on residential driveways where the customer tried using multiple products from the hardware store, including Muratic acid).
Since the glue or mastic has been adhered to the concrete for many tears, is it embedded. As suggested above, if your customer is expecting new looking concrete, without all the checkerboard squares and glue left behind, a floor grinder/resurfacer is your only option.