As long as he's willing to accept it WILL take longer with an older CPU, there is no need to upgrade. If the system meets their needs for everything else they're doing, they can probably stand the wait.
It all depends on what it is he/she is trying to do. Which I gather is a DVD9 -> DVD5 transcode. Which is seldom a good thing, quality wise.
Set it up and let it run overnight, and time won't matter while you're sleeping. And you can save $1000 by not upgrading for now.
I'm still using a P3 1ghz and can transcode is under 5 hours while I'm sleeping. Or I can use DVD2ONE (or Nero Recode) and just strip the main movie file(s) in about an hour. The wife & I can't afford to spend $1000 to upgrade. The computer works fine for every other task we put to it.