I'm 5'4 and use a 50 cm road bike, but I have short legs and a long torso. I think my bike is considered "medium" size for women. It really depends on her inseam and build. Does she have long legs/short torso or the opposite? Women with long legs/short torso can benefit from WSD (women specific design) bikes. Women with short legs/long torso may be fine on men's bikes. Arm length is also an important factor for women's bikes - some women require shorter handlebar length or narrower handlebars. I would recommend asking someone in your family who would know what her inseam is, and then describe that and her build to someone at your local bike shop to get an idea on size.
Are you sure you don't want to buy from a local bike shop? If you do, then you can take her and the bike down there and they can adjust everything so it fits properly - that makes it more comfortable and less likely that she'll injure herself. And they can give you good hints on components.
Good luck!