I'd say this person could use firefox as it should hypothetically be able to detect the character encoding. If you.. uh sorry, I mean this person needs to be able to edit the files the free Japanese word processor JWPce also supports all sorts of encoding and should be able to convert. (EDIT and off course, JWPce is not available on the Mac so that won't work 
Edited: 2010-02-19, 3:37 am