The brief
The A8 had developed squealing and grinding when the brakes went on, the stopping distance had stretched, and the pedal felt a little soft. He brought it in. Squealing and grinding when braking, with weaker stopping, says the pads are worn. The pads are the friction material that does the actual stopping, pressed against the discs, and they wear down a little every time you brake. When they get thin, a small metal tab built into them starts touching the disc, which is the squeal, and once they're past that point the grinding starts and the stopping suffers. Pads are a wear item, and on a big, heavy car like this you don't let them run out, so it came in for a fresh set.
The diagnosis
Wheels off, the front pads were worn down to the wear indicator, done. The discs were checked and measured, and they were still within spec with a smooth surface, so they didn't need changing this time, just the pads. So it was a front pad replacement, with the discs cleaned up and confirmed good.
The work
The front callipers came off, the worn pads removed, and a fresh set of genuine Audi-spec pads fitted, with the slider pins cleaned and greased so the callipers float freely and the calliper bolts torqued back to spec. Then the new pads were bedded in on a controlled road test, a series of measured stops to lay an even layer of pad material onto the discs. The road test confirmed the squeal and the grind were gone, the pedal was firm, and the stopping was back.
The outcome
No squeal or grind, a firm pedal, and full bite back at the front. The A8 went home with the front brakes fresh and the discs confirmed good for a good while yet. Pads are a wear item and there's no nursing them past the indicator, so a fresh set put the stopping properly right.