I looked at Raleigh, Trek and Giant and Trek has the best range of bikes in the lower range.
Giant has a really sweet looking bike for 500 bucks, i don't think she's going to want to spend that much; Raleigh makes a nice bike too; but their sizes are a bit odd (they have 2 "womens" bikes and they have bigger sizes than the others)

Her goal is to ride the STP (Seattle to Portland)
It's a lofty goal considering, but I'm sure if she works at it she can do it.