Upgrading from 4 to 8 gigs last week helped a lot. I'd go to 16 except her MBP won't support it.
I'd love to get her on an SSD but she's on a 1tb drive now and it would be hard for her to try and fit into a 512gb SSD now (especially now that she's on the D800 with huge video files and 72mb raw photo files.
It's frustrating that it will work find some of the time and not others, implying that the problem could be fixed with better memory management. I do hope that a serious Adobe competitor arises to force Adobe to make its apps faster and more resource efficient.
Forget the advice. Lightroom is faster, as noted in every review of both programs. Try Aperture yourself with the demo to find out.
Working with 10+ megapixel images is always going to be slow, and with camera advances, it will get worse every time your wife gets a higher resolution camera --so comparing it with how it used to be when you have 6mp files is not exactly correct.
More memory and an SSD will definitely help.