If you're sure the pond is leaking from the dam or from beneath the dam, then I think digging a trench down to good clay soil beneath the dam, filling it with clay, compacting it as you go, would work. You'd need to carry that clay out past the end of the dam, tieing it into good clay all the way on the hillside at each end of the dam, above the high water line of the pond.
If you're not sure where it's leaking then plating and compacting clay over the whole pond basin up past the high water mark is the route I'd take.
If he says "What I propose to do is what will stop the leak and if it still leaks I'll come back and fix it." then disregard what I said and go with what he says.
