If the sink did not have it's own trap, then there would be no reason to vent it. If that were the case it would be sharing the washer's trap (which must have it's own vent). I have no idea if that would be allowed where you you live.
The type work you are planning requires a permit from the local municipality. I recommend you go speak to your local plumbing inspector and see what he thinks of your plumber's idea.