Spending £40 more for a system that'll last longer wont really be the case if he's going to overclock the nuts off it Which he won't based on what hes said...
And besides, just because its C2D theres no reason it'd last longer. If it were me i'd go for the C2D system, but based on their needs id go for a 939 type system and save the cash.
For someone who's just using the net, and word processing shes never going to need a more powerful pc....ever