Munky - yeah probably not the best car considering its old as well. I mean, the engine sounds ok. In the end though, if your gf is happy with it, then its her decision I suppose. A revs check might be worthwhile if she does infact want it. Usually these mx5's go for around 12 grand if they are in good nic and they hold value - so yeh the fact that they had the car for a year and selling it for 9 grand might hint that there could be some problems with it.
If your gf can afford to spend a bit more, maybe look around for something thats in a better condition (that is if she still wants an mx5).