The BB on his bike will be pretty standard, it'll have to match the chainset rather than the bike and we don't know from your post what that is. I only know about square taper, it's quite likely to be that type but Thorn have used others. It needs to match the frame BB width (EDIT - mickeg points out the MkII is 73mm, likewise the MkIII, I don't know about previous models), and the spindle length (You can get away with +/- a few mm on the spindle) ST used to be ubiquitous, they're not so easy to find anymore, particularly if they need a 73mm width, though that availability may vary around the World.
Square taper do fail, often water getting past the seals, but it's rarely a catastrophic failure. You'd have to have ignored the warning signs for hundreds (Maybe thousands) of miles before it became unusable. If it's at the early stages of failure - the bearing feeling or sounding rough but the cranks still spinning fairly freely - then injecting some grease under the seals into the bearings can prolong the life, though it isn't a permeant solution.
EDIT 2 - Has anyone experience of running a 68mm BB in a 73mm shell? I have a vague recollection of reading about someone doing so, though it might not have been here.
