I have seen similar Dell's and they too run at 333 regardless if whether 400Mhz DDR is installed. May be a chipset limitation, but likely a self imposed limitation designed by Dell to ensure maximum stability. Love 'em or hate 'em, but Dells in their untweakable configs do tend to last forever.
Loaded questions there. Do vendors take shortcuts....yes, but that would be a dissertation in and of itself. In this case, it is hard to tell.
One issue we didn't discuss was how some older DDR memory based systems had some built-in speed limitations, they would clock back from 400 to 333 when memory was maxed out. This limitation is tied to memory standards (or the lack of) when DDR was king of the memory standards.
Because not all vendors applied the standards correctly, vendors would clock systems back to 333 to ensure stability over the minor increase in performance at the 400 level. On "generic" motherboards, users could manually enter the BIOS to reconfigure back to the higher speed settings.
With your daughter-in-law's Dell, as you know, there are very limited BIOS options and I am sure memory speed in not modifiable. In Dell's defense (I don't say that often), this is probably the case with this system.
Anyway, you are a good man to do this for her. HOOAH!!!