The brief
Cold-start squeal was the first sign. Then the belt started slipping under aircon and alternator load, with that familiar high-pitched noise. The Passat was past 120,000 km, so the owner correctly suspected the belt and brought it in.
The accessory belt is the one that drives the alternator, the aircon compressor and the power steering off the front of the engine, kept tight by a spring-loaded tensioner running on a pulley, with an idler pulley to route it. As the belt ages it glazes and cracks, and as the tensioner and idler bearings wear they stop holding the belt steady, which is the squeal on a cold start and the slip under load. A belt that's started doing that on a high-mileage engine is a set on its way out, and a snapped accessory belt leaves you with no charge and no steering assist.
The diagnosis
The belt itself was glazed and showed fine cracks along the rib face, classic age-and-heat fatigue. The tensioner had a small wobble on its bearing, and the idler pulley spun rough.
Doing just the belt would leave two worn parts ready to be the next problem behind it, so it was a set job, belt, tensioner and idler pulley together.
The work
The tensioner was released, the old belt slipped off, and the tensioner and idler pulley swapped for new VAG-spec parts. A fresh belt went on, routed correctly through every pulley, and re-tensioned to spec.
A road test confirmed the squeal was gone and the belt drive ran silent.
The outcome
No squeal on a cold start, no slip under load, and no noise from the front of the engine.
The Passat went home with the belt drive back to silent and ready for another long stretch. The accessory belt and its pulleys wear together, and a snapped belt strands you with no charge and no steering assist, so doing the set on the first squeal kept it to a tidy job.