I finally got in touch with her yesterday and she wasn't able to obtain an official Apple opening tool at the time, so I decided to hunt down the necessary widget myself, which happens to be a regular 4" putty knife. Which is about $4 at Walmart.
It's actually fairly tricky to open the Mini, because there are clips on three sides that need to be unlatched with the putty knife before the case can be opened. Once you unclip one side with the putty knife and pull the case apart a few millimetres, two things happen: (a) the case has a strong tendency to snap back together unless you wedge it open with something, and (b) the opposite side becomes even more tightly wedged together which makes it hard to get the putty knife in there. Once you unlatch the two opposite sides though, the third side comes apart without having to use the knife.
Once open, the memory module is blocked by the Bluetooth adapter (if installed), which comes off after loosening the screw holding it in place (you don't want to remove the screw all the way). The memory is held in place with normal latches on each end, but the one at the rear of the machine is slightly blocked by other components so it doesn't open all the way. It was still easy enough to remove the 256M module and insert the 1GB module.
Reassembling the unit was a bit trickier than opening it, because a couple of metal prongs on each side at the back had to be coerced into not binding against the plastic case. Once that was done, the unit slipped easily back together.
By selling the Mini with only 256M of memory, Apple is doing themselves a disservice. That is the absolute minimum amount of memory required for Tiger, and the machine is almost unusable with such a constraint. For example, Amy couldn't even listen to iTunes while using Photoshop Elements at the same time. With 1G, the machine is much more responsive and is no longer memory bound.
So, if anybody with a Mini wants to borrow the putty knife, let me know.