The brief
The Golf had its ABS warning light on, sometimes the traction control and stability light with it, and the systems kicking out so you were left on plain brakes. He brought it in, which is the right call, ABS and traction control are safety systems and a warning means one of them is offline. The usual cause is a wheel speed sensor. There's one at each wheel, and they tell the ABS module how fast each wheel is turning so it can step in if one locks or spins. The sensors sit down by the hubs, exposed to heat, water and road grime, and they fail, the element goes, the wiring corrodes, or the tone ring they read gets dirty or damaged. When one stops giving a clean signal the module can't trust the data, so it shuts ABS and traction control down and lights the dash. A failed sensor doesn't recover, so it needs replacing.
The diagnosis
A diagnostic scan pulled the fault straight to a wheel speed sensor on one corner, no clean signal coming from it, which is exactly what trips the ABS and traction lights. The other three sensors and the tone rings were reading fine, and the brakes themselves checked out, it was that one sensor. That's a sensor replacement on the affected corner, you don't repair a failed speed sensor, so the call was a new genuine sensor, fitted and the codes cleared.
The work
The wheel came off, the failed ABS sensor was removed from the hub, the mounting cleaned up, and a new genuine VW-spec wheel speed sensor fitted and routed properly so the wiring's protected. The system was scanned to confirm a clean signal from all four wheels and the stored ABS and traction codes cleared. A road test confirmed the ABS and traction control lights stayed off and the systems were active again.
The outcome
No ABS warning, no traction or stability light, the systems back online, and a clean signal from every wheel. The Golf went home with the safety systems working again. A failed wheel speed sensor takes ABS and traction control offline until it's fixed, so changing it on the affected corner and clearing the codes put the braking systems back where they should be.