yes those crf150's are sweet, I was watching some kid out at the track the other day on one and they sound like my 250f.  The only thing is, they might not be able to get green stickered so she will be restricted to tracks in the summer and only be able to ride trails in the winter, that's why i suggested a trail style bike.

Edit: I read what skid had to say about the crf230 and that may be true as I do not know AS much about trail bikes as i do about performance bikes.  I just know my yz250f has no problem taking my 160pound 6 foot tall ass over a 110 foot double as i was doing las weekend.  I figured the crf230 wouldnt be a slow pig but have not be too fast for a beginner.  Personally if I was buying my mom a bike and 2 strokes were still ok to ride on trails I would get her a kx100 2 stroke.