He must not be doing the install correctly. When offered the location for the pgm. to be installed at, browse for the HDD and choose it instead. Sometimes you may have to "add a folder", although most of the time just choosing the HDD will be enough.
He's in fact choosing the correct HDD, but soe some reason, game update files want to go into the C drive for some reason. Here's another thread I found with a similar problem, yet we tried to follow those steps and so far have moved over his Program Files folder to the other drive, but are unable to actually then delete the old files, so we didn't end up saving any space.http://www.tomshardware.com/forum/262429-32-stop-programs-installing