I just have a few random thoughts...
Have you talked to her about it? What does
she want? Even if she doesn't know a KLX from a KDX, I guarantee that if you take her around and show her a CRF230L, an XT225 (better than the 250 IMO) a KLX, etc., one of those bikes is going to strike her fancy. She is, for sure, going to like one of them much better than the other. We might know for a fact that the red bike is waaay better than the blue one because it's 2.6 pounds lighter and has better footpegs and more top end, but she might like the yellow one better because it's more comfortable and much better looking. You want her to be happy, and you want to cultivate her love for the sport. This is most easily done when there is a bike in the garage that she is genuinely excited about. Of course, her choice has to be somewhat reasonable (I like that orange one that says 950 SE on the side, honey!) but if it's within an acceptable range, go for it. I speak from a ton of "my girfriend wants a bike" experience and I've seen the same thing over and over and over.
Another thing to think about is the fact that this will be her first bike and her tastes will probably change once she gets some saddle time. She might decide she likes the street side of things better, or she might want a more dirt-oriented bike for trails, or she might accidentally stumble onto a motocross track and from then on nothing but a 125 2-stroke will do. Just as most kids bikes (PW50's for instance) go through many owners, there are lots of beginner-type bikes out there that are being sold with very little use on them. Consider a used bike until she figures out what she really digs about riding. It's much easier to bail on a bike that was bought used than one that has been heavily invested in. I knew a gal that kept riding a bike she hated (albeit not very often) because she felt bad about her boyfriend spending so much money on it and she didn't want to disappoint him. Don't let this happen to you.
You're already ahead of the game by finding a tall girlfriend and by choosing to
not be the one to teach her how to ride

Take her around to as many shops as you can and have her sit on everything that fits the criteria.
Oh, and don't forget about the DRZ200. It's $1100 less than the KLX and weighs 20lbs less, too. 20lbs is
huge if she really wants to ride it off-road.
Good luck!
WoodsChick