At 5'7" she is in a fairly common size range. Advantage YOU because there are alot of bikes out there used, to borrow, to rent, etc., in that size range.
I would suggest borrowing or renting a bike before you buy one. And rather than a pure road bike, I would look around for a cyclocross or "gravel" bike. The geometry is a little more slack and the ride almost always more comfortable, especially with wider tires and they are usually equipped with disc brakes (which I entirely dig). Something like these...
https://www.performancebike.com/shop/bikes-frames/road-bikes/gravel-road-bikes?cm_mmc=Google%20CPC-_-Dynamic%20Search%20Ads-_-Bikes%20&%20Frames-_-DSA_Bikes%20&%20Frames#facet:&productBeginIndex:0&facetLimit:&orderBy:5&pageView:grid&minPrice:&maxPrice:&pageSize:&
In this price range, if you are buying new, it will likely be aluminum (though the link I gave you sported a few Breezer steel frames). It is light and strong. Also tends to be stiff as a board. Tire size/selection makes all the difference in the world on an aluminum frame. A frame with enough clearance for wider, lower pressure tires turns a buckboard frame into an entirely pleasant bike.
If you are buying second hand, the market opens up substantially to nice quality steel frames and even some carbon fiber. A quality steel frame is surprisingly light and rides like a dream (a cheap one feels like it is made of water pipes). Carbon goes all over the place from extremely stiff to highly compliant depending on how the bike is built.
angela