Autodesk previously only recommended professional cards for use with Maya, but they've become much more friendly toward consumer/gaming cards in the past few years (especially with the new viewport and DX11 support). I expect this trend will continue given how much they're investing in the Stingray engine and inter-op with Maya and Max.
With that in mind, I don't think it's worth the money to purchase a Quadro or FirePro. For that matter, I don't recommend any AMD cards whatsoever. We have to use them at work and they're very capable at running the game engine, but every year it seems like we run into driver issues with apps like Maya. For that reason, I'd stick with a GeForce card.
What kind of work is your girlfriend doing? Lots of high-res modeling? Animation? Any GPU rendering with stuff like Octane or V-Ray RT that would require tons of CUDA cores and memory on the card? Maya is a tricky beast in that throwing hardware at the problem won't necessarily improve performance. For example, slow animation playback can be the result of Maya's poor multi-threading for rig evaluation, or because the environment geometry needs to be baked into an Alembic cache. So many variables :/
That said, I'm running a 660 Ti 3 GB at home and don't have any issues. If your budget allows for it, something like the GTX 970 is probably a good fit (especially if she plays games or needs to run UE4/Unity). I don't think spending any more money will have tangible effects in most people's Maya workflows.