I would suggest a Yamaha TTR125 LE (or TTR125L). It's a great trail bike for a beginner, the LE has an electric start (L doesn't), it's a big wheel model, and you might be able to pick up a brand new one in your price range. Bike weight is around 170 lbs. without electric start and 185 with the electric start.
http://www.yamaha-motor.com/sport/products/modelhome/237/0/home.aspx
The Honda CRF100F and CRF150F are okay options too, but the CRF100F has a little less motor, same size tires but slightly smaller, weighs about the same as the TTR, and I don't think they offer an electric start. And the CRF150F has a bigger motor, same size tires and a little taller bike, offers electric start, but weighs in at 225 lbs! That's a heavy bike for a beginner.
A CR80/85 2-stroke is usually difficult for a first bike because the power is like a lightswitch... one minute you're boggin, next it's a rocket! So it's a little harder to learn the clutch, and to control the hit and ride smooth on a 80/85cc 2-stroke for a first bike, especially in the woods. The little four-stroke trail bikes are more mellow and smoother with the power delivery. They require less clutchwork and are easier to ride for a beginner.
Good luck!! :ride:
ditto , got ttr or i would say crf100. you dont want her over bikes and a 100 would probably be plenty. most likely she wont be doing much other than putting around. my gf rides a little pw50 and she is 5'6" about 130. went out to a big dry lake bed which was great. usually the wifes are putting, not doing the crazy trails. i would give a negative on a 2 stroke for her