What skill level is she? What type of riding does she like to do?
I'm still beginner/low intermediate and picked up a Ride Rapture (the 2011 version from Evo outlet store) this year. It's my first board and I'm quite happy with it, but the only thing I have to compare it to is several trips worth of rental boards (it's much better than those). It seems reasonably durable so far, with only a few minor scratches to show for having ridden over quite a few stumps and rocks in my 4 trips out so far this season.
I'm 5'6" and 115lbs and got the 151 but would probably get the 147 if I had it to do over again. Even so, I haven't found that I feel like I'm really having to manhandle the board the way I often did with rental boards. For the price (~$190 after coupons) I think the Rapture was a great choice.