Well if he is fixing the sink he is more or less going to be fixing the tap if he uses his sense anyways, you'd not employ him to do one and not the other.
no; not if you hired him to fix your blocked sink --- just because he is a plumber doesn't mean he has to fix your tap that is faulty as well.

And, by the way, he did what any sensible employee would do --- shopped his cv to one the best firms around and got the wages and respect he feel he deserves

He is not the one moaning about loyalty.