The brief
The A3 came in with the brakes worn, squealing under braking and the stopping gone weaker than it should be. He brought it in before it got worse. Squealing under braking means the pads are down to or past their wear indicator, and weaker stopping is the friction material running out. Brakes are a wear item and there's no nursing them past the indicator, so when they get to that point the pads and the discs go together, because the discs wear with the pads and fitting fresh pads onto a worn disc just beds them into the same uneven surface. On a regular service the brakes get inspected for exactly this, so they're caught before the grinding starts.
The diagnosis
Wheels off, the front pads were down to the wear indicator and the discs had a lip around the edge and measured past the minimum thickness on the gauge, so they were done too. Fitting new pads onto worn discs doesn't make sense, so it was a set job, front pads and discs together, to reset the braking properly.
The work
The front callipers came off, the worn discs were swapped for new genuine Audi-spec rotors, and a fresh set of pads went in. The slider pins were cleaned and greased so the callipers float freely, everything reassembled to torque, and the brakes bedded in on a controlled road test, a series of measured stops to lay an even layer of pad material onto the new discs. The road test confirmed the squeal and the weak stopping were gone, the pedal was firm, and the bite was back.
The outcome
No squeal, a firm pedal, and full, even bite back at the front. The A3 went home with the braking reset to like-new. Brakes are a wear item and there's no nursing them past the indicator, so doing the pads and discs together gave the car back proper, even stopping power.