For the heat distribution via steam radiators you need a steam boiler (which is not technically a furnace.)
The high boiler temperatures required to make 215F+ steam result in higher stack temperatures (= a greater fraction of the heat going up the flue, which means a lower raw combustion efficiency), so you'll never hit 90%. But 82-83% is do-able with natural gas (you can do slightly higher combustion efficiency with oil, maybe 85-86%, but that's it), and that's where the better steam boilers live these days when new, with both the boiler & system dialed-in.
To hit 90% the exhaust temp has to be WAY under 200F, which takes return water from radiation to be under 125F. That simply isn't possible with 150-200F water returning to the boiler from the radiators. There is no such thing as a "condensing steam boiler", except those where the cool makeup water is being pre-heated by a secondary condensing economizer. SFAIK those systems that exist are limited to commercial boilers, not residential systems, and even with those hitting the 90% number requires some system design optimization. It's conceivable that your friend has a system large enough to handle the output of a smaller commercial steam boiler with a condensing economizer for preheating the make-up water, but it would be unusual for residential systems.
It's possible to retrofit an oil-fired steam boiler with a gas "conversion burner" at a reasonable price tag. If it's a decent oil boiler you'd be able to hit ~78-80% efficiency with a gas conversion burner, but if it's an ancient asbestos covered pig that had been converted from coal to oil you'd be lucky to hit 70%, and it's worth considering an all-new boiler.
American Standard steam boilers have been around for a long time, but I'm not sure they're still making steam boilers under that name. The Burnham Independence series seems to be a popular replacement boiler in my area, and can hit 83% when tuned. The Weil-McLain EG series is pretty similar and also hits the 83% range when dialed in. I'm sure there are others.
The high temperatures and large surface area of steam piping also requires a decent R-value on all steam piping (including the return plumbing) in order to hit a reasonable system efficiency. Retrofitting 1" wall-thickness ~R4 fiberglass or rock wool pipe insulation not cheap, but if the boiler room is the warmest place in the house during the winter a large fraction of the heat is being wasted, and even at buck-a-therm gas there is real payback on it. The goods sold at box stores is usually 1/2" wall thickness or ~R2. It's worth going online or to a real plumbing supply distributor to get the 1" or thicker stuff, which won't be much more expensive than box-store pricing on the thin stuff. If you can't find it locally, try these guys.