I would recommend the minimum processor at 1.6GHz (Intel). That is the line where AV captures to DV can occur at 30fps according to a source I can no longer quote (it has been about 3 years...)
As noted, find the software and work from there is the best approach. Does she have a digital video camera? I would consider:
Better than 2.4GHz or equivalent
At least 512MB mem.
2 drives, 60-160GB OS; 120-250GB for video
Pioneer 108 or NEC 3500 DVD drive
Firewire (IEEE 1394)
Optical mouse (Lots of dragging - if the mouse tracks funny, she'll hate it)
Something other than on-board video (9600+ or such).
Work up from there. If she does not have a DV camera, look into something like Pinnacle's Studio 9 with the AV card or better still, the Moviebox DV (the DV works better than the USB version according to some posts I have seen - I don't do much AV and use an All-In-Wonder when I need to).