I will be replacing tires soon on my standard 17 inch rims - the Euros have 98 load rated tires as standard (both standard and Lux model. I want to drop from 225/50/17 to 225/45/17 (I don't mind shortening the gearing a smidge), but can't find any 225/45's rated over 94.
Tire fitters and insurers won't go for anything less than 98 even though it might be technically safe to go with 94.
As Tony said, 50 would be the profile, that is the "thickness" of the tyre as some would put it, or the height of the sidewall. However, if he keeps the rim diameter the same, and only reduces the profile, his speedo will be off. If his doing it for the looks, then don't. You're car will look like a 4wd, because the gaps in the arches are bigger. Actually, it'll look more like a shopping cart because the wheels are so small.