I don't know much about oil costs since everything out here is natural gas, but if he got a fairly high efficiency boiler it shouldn't be any less efficient than a condensing natural gas boiler used for space heating. The reason for that is depending on how hot the boiler control is set at, it would pretty much eliminate the gains from a condensing boiler. If your water supply is hotter than 140-150F, by definition the boiler flue gases can no longer condense, which is what boosts the efficiency up in to the 90's. The less heat you throw out your chimney, the more efficient.
And boilers by nature are "tankless", although some do have a fairly high mass (water content). I would assume though that the boiler he got is a low mass, fire tube design that might have a heat exchanger with a water content of 10-20 gallons. High mass boilers are pretty rare in most small and medium sized applications these days.