Hi
I have been working on one of my cars today .. a Renault Megane Coupe .. working on the ABS!
To change the shaft, they 'should' have removed the sensor from the hub carrier .. but many dont. So they must have torn the cable or damaged the sensor. The reluctor ring is famous for cracking .. so you need to know if shaft was new .. remanufactured .. or a fix-up job. Reluctor rings are on eBay alone for about a tenner. Get the correct no of teeth if you need one. They need heating to fit on (shrink fit)
When the sensors are correct, the ABS light .. and the Service light will go out of own accord down the road. The car ECU stores minor codes .. but will not stop ABS working.
Mark1 facelift and Mk2 have electronic speedo fed from ABS unit. This takes a signal from any wheel sensor input.
Get them to replace/repair the sensor, and make sure replacement match original (especially the ring teeth no)
Any more info please ask! : )