I don't shoot a classic double, but that stuff is I believe 18 gr. something or other, thus very heavy on the tungsten so is probably too hard, others might have a better insight on it, but he should be able to find bismuth, and Ballistics products has a tin plated bismuth shot for reloading. That would work in the old doubles. Other then the shot, a lot of the old double guys recommend keeping the pressure down also. Then there is the issue if the chambers are a true 2 3/4 or shorter, if the gun has the shorter chamber, the pressure can rise with the 2 3/4" shells.
Then, to make it more complicated, lol, even if the steel in the barrels can take the pressure, often the stock can't, due to oil saturation, high pressure, and just age.
I looked into getting an old Fox, but unless your shooting the recommended shells it just didn't seem worthwhile to me...YMMV.