There is no question a quad core is better. I wouldn't be stupid enough to suggest that. What I'm saying is it really isn't a necessary upgrade basic video editting.
Not windows movie maker, either. If you are using Premiere or Avid a high powered dual core will do you fine. If you're using Maya or Lightwave, or heavy after effects, then sure, the rendering time will bug you alot...but just editting video? Standard wipes, scrolls, transitions, ect, are not going to take that long to render and won't need to be rendered very often.
Of course a quad core is better, but its not really necessary. The guy is asking for a cheap and easy upgrade and you are trying to tell him to build a brand new, very expensive, system.