What could cause smelly water in the bathroom sink only?
Just a few suggestions; I find it hard to imagine that it really is the water that smells, so also check the following:


Most obvious is that sewer gas is coming in somehow. Check the p-trap for leaks, make sure there is always water in it. If you don't use the sink often, the p-trap water can evaporate.
Check the ventilation of the sewer lines. Could there be some obstruction in it? Assuming the smell started when the work was done, did anything else change in the system. Is it possible the vent was closed off or obstructed by mistake?