I'm surprised nobody has suggested signing your girlfriend up for an off road motorcycle class first before buying a bike!
When I announced to my husband that I wanted to ride, he enrolled me in a one day class that HONDA offers. I'm glad I took the class for a few reasons: 1) it boosted my confidence 2) made me realize the CRF150 was going to be way too small for me (I'm 5' 6 1/2", 140lbs.) and 3) I wanted an electric start.
My first bike was a used CRF 230, which I rode for a little less than a year. I definitely had a great time on that bike. It is a little heavy and the suspension, well there is no suspension, but it was a good bike to start on. When it was time to sell it, we had no problem. I think it took less than 2 weeks.
I recently bought a brand new 2006 CRF250X. My husband went all out for me: put an FMF pipe on it, had it rejetted, and the suspension done. At first the extra power freaked me out a little, but now I don't even notice it other than I go faster. This is probably the bike I will ride for a very long time.
So I say, find out where the nearest off road motorcycle class is and sign your girlfriend up. I think it will help take the guess work out of what to buy.