The Mansfield manual recommends a high burn once or twice a day, and states that owners can see up to 600 on a high burn, an 300-400 on a low burn. I own a Heritage, the mid-sized version of the Mansfield, and the manual says that high burn temps run up to 500, and low burn temps run about 200-300, and to avoid running the Heritage at sustained temps over 600. I didn't see a specific number in the Mansfield manual for overfiring, nor how running a blower will affect those numbers.
Based on that, I'd say that depending upon how `+' that `600+' gets, then yes, he's running it at the high end of what's recommended, but possibly not in the overfiring range.
I've never lived with a Mansfield, nor known anyone who has, except through this forum. I don't consider the Heritage to be a slow heater for two reasons: first, when I run it for steady heat when it's cold outside (-0F and below), the stove is pretty much warm 24/7. So even when I come home and the fire has been out for awhile, my stove is still warm to the touch. It doesn't take long to get the temps back up from 150-200F.
The second reason is that the large glass front throws off a lot of heat. I come home, start with the fire, and just move between my other chores and tending the fire. By the time I'm ready to pull up a chair in front of the stove (about 20 minutes), it's time to turn the air intake down on my run-up fire. The stove glass is throwing out serious heat at this point--the kind that makes you slide your chair back--and then back--and then back--and the stone top and sides are warming. At this point, the thermometer in the hearth room is starting to push 70 (up from 62-65, usually) and the heat by then is starting to distribute whole-house, so the hearthroom temp doesn't get a lot warmer unless I want it warmer or get lazy in my fire-tending.
I think one thing that helps is that the heat from the stove keeps the walls, floor, and furniture warmer, so that I'm not overcoming that cold-sink sensation. One day my gasket failed and my fire burned out faster than normal, so I came home to a house that was cold to the bones. It took awhile to fix the gasket, with some help and moral support from a forum member, and quite awhile to get the house warmed after that. (Thanks gyrfalcon--you're the man!)
On mild days like it's warmed up to now (0F to 20F), if it's sunny out I only need one fire a day to warm the house, and can anticipate that curve, have a fire laid, and light it off as it starts to chill.
That having been said, this is the Heritage, not the Mansfield, and not Woodstock stoves. The larger Hearthstone may have a much longer warm-up time, and the double-walled soapstone stoves that Woodstock builds, I assume take longer to heat up. Both of these stoves then would take even longer than mine to cool off, thus evening out the temp fluctuations.
The question I'm not seeing asked here, and what puzzles me, is why your friend would need to run the stove that hard.
You say you're in New England, which seems to me to be the high-quality stove wood capital of the universe. Assumption on my part is that he's burning optimal wood. I'm getting the results I described with wood that another poster here dismissed as "garbage": aspen and cottonwood.
Another assumption: he has a very large house, or he wouldn't have picked that stove in the first place. For comparison's sake, I have a modest sized house (thus the Heritage)--about 2000sf--and can heat both floors of my house pretty evenly while running this stove at cruising temps. I don't think I've ever run the stove overnight on anything other than a closed air intake. I do the run-up fires daily, then usually have the air intake shut down for the rest of the burn cycle, even have a stack damper engaged for most of it (not the burn-out fires, obviously).
Assuming a house in the 3000-4000 sf range, he may be wise to consider putting another stove in somewhere on the other end of the house rather than run the Mansfield full-out. Yet another assumption: he's not running anything else for supplemental heat in this house. He could work on other ways to deal with the cold. You mentioned windchill being a problem, which suggests to me that he might benefit from some thoughtful landscaping. Strategic placement of a woodshed, fence, or coniferous trees can help a lot. Also could look at insulation, skirting, new windows, etc. It may even be wise to turn on supplemental heat--boiler, furnace--on the really bitter nights.
There are soapstone stoves still in use that are several decades old--these were the inspiration for Woodstock stoves. I hope my stove will last my lifetime, and beyond, and I use it accordingly.