craigslist. With your help she can get a nice, used, entry to mid level hardtail off craigslist.
The other option is to try REI during a sale. Their entry level mountain bikes start at $350-400 but if she can tack on 20% off, a member discount, and a sale price it might get down into her range and be ok for light trail riding. She'll get a better bike if she has some patience with craigslist though.