I'm a Canon guy, so I admittedly don't know a whole lot about comparable Nikon cameras, but I, personally, would stick with Canon and Nikon for DSLRs solely for the used lens market. Lots of people selling slightly used lens from both families, and whenever you can knock a couple hundred bucks off the price of higher-end lenses... that's nice. However, not sure how keen she is on continuing photography in the future so that may not matter.
When you say "intent on doing low-light/slow-exposure kinda stuff", do you mean that's the style she's looking to shoot or that she wants to shoot in low-light situations that would normally require a slow exposure?
Canon has lots of solid low-priced DSLRs. A T1i with kit lens would probably suit her just fine if she's just looking for long-exposure shots. When I got my first DSLR, it was a Digital Rebel XT with a 17-85mm F4-5.6 lens. It did everything I needed it to do just starting out, including long-exposure night photography. Again, I don't know the models for comparable Nikon bodies, but they'll do well for you, too.
If she's looking for something that lets in a little more light to be able to avoid longer exposures and higher ISOs, maybe invest in a cheap 50mm F1.8 and she can go up or down in focal length from there to suit her needs.
The newer you go and the bigger the sensor, the better low-light performance the sensor will have, generally. My XT at ISO 1600 was... passable. My 7D at 1600 looks solid... of course, that's a difference of about 5 years in technological advancements
Just my two cents
I'm sure many others can/will recommend different.