If I were going to spend $5000 on a bentonite fix, I would "bite the bullet" drain the pond, let it dry enough to drive a tractor with roto-tiller over the pond bottom. Work bentonite into the top 6" to 10" of bottom soil (scarify). Compact well with a sheep-foot roller and refill. That whole operation would not cost more than $5000 and probably much less say around $1000-$2200. The chances of the pond now holding water are much, much greater than the "other way" (water application). Lack of guarantee should make you suspect. If you use his method ask for references and check them out for long term seal.
With a bentonite seal it is VERY important to keep the pond full so the soil layer in the dried zone does not crack and allow water to escape.
