I may be far too late to this thread, but I'd second
@stringer suggestions and the scimitar blade pattern he attached. As a butcher, my preferences for an order like this would be very little/no flex, a fairly thin-convex grind to limit sticking (while still maintaining limited flex), monosteel (you should probably ask your customer about his sharpening methods if you haven't already, because if he predominantly uses a steeling rod, you probably want to adjust the blade steel/treatment accordingly). The tallest meat slicer I have is 50mm, and I actually prefer that 45-50mm range as it keeps my hand away from the cutting boards/bones/etc and allows me to finish a cut smoothly at the heel of the blade if I need to without having to change the angle of my wrist or cut (it may also be what he's used to since french style butcher knives tend to be taller than their american/japanese counterparts). As for the handle, probably a western handle (judging by the french patterned butcher knives he has in the photos), but it may be worth asking him if he ever cuts meat in reverse-grip, because some styles of western handle that aren't a neutral shape can make that a little more awkward.
I'm assuming the extreme length is to allow for an easier single cut and keeping his hand well away from the bones, since 500mm is definitely not necessary for cutting bone-in ribeyes. Maybe I'm biased though, since I'm one of those butchers who thinks tomahawks are just a trendy, overpriced cut and a waste of good short rib. I will admit they do look mighty impressive though.
Let us know how you get on and what you come up with!