more ram allows a larger bucket size which (usually) allows faster renders.

it also speeds up your overall workflow/desktop experience if windows isnt chugging away reading/writing back and forth from your pagefile constantly.  in normal everyday use i dont notice much of a performance gain going from 8 to 16 gigs, going from 4 to 8 did make a difference.
when doing heavy multitasking with poser, photoshop, silo, bodypaint, etc open all at the same time the difference between 8-16 is night and day. it also means i dont have to worry about having firefox and a dozen tabs open, etc. its nice not to have to juggle programs.  when rendering hires maps in Zbrush its nice not to have to kill every single process thats running so i dont get out of memory errors.

DDR3 RAM is dirt cheap right now. no, its not going to make his computer twice as fast, but its a no-brainer upgrade unless you are on a very tight budget.

all this is, of course, assuming he is on a 64 bit OS, hopefully with Poser 2012.