It isn't technical mountaineering most of the time (although in late autumn, winter and early spring it can be), but it is a huge altitude gain, is bloody steep and you do need to be fairly sure-footed, especially coming down where if you slip in the wrong place, the results can be rather serious. You also need to have a few clues about keeping to the marked route (the upper part of it is a route, not a track). There have been two cases in the past year where overseas tourists lost the route on the way back down and headed down the wrong way, then fell a vast distance down towards the Mueller Glacier a couple of thousand feet below. Neither of them survived it. I'm not trying to put your hubby off it or anything, but he DOES need to be aware that it isn't a casual stroll in the park.
Charlie Hobbs would be able to fill you in with a lot more information about it, or you could go to the Department of Conservation website and search for
Mueller Hut route. There is probably a lot of useful info on there somewhere.
http://www.doc.govt.nz/ Or, check out the following webpages on the DOC website (the second one has links to a route brochure in two parts)....
http://www.doc.govt.nz/parks-and-recreation/tracks-and-walks/canterbury/aoraki-area/mueller-hut-route/track-description/http://www.doc.govt.nz/publications/parks-and-recreation/tracks-and-walks/canterbury/mueller-hut-route/