It also depends on the torso/leg ratio. I'm 5'7" but have a 53cm bike because of my disproportionate legs. And like has been posted above, different brands fit differently, even at the "same" size. My best advice would be to have her go to a bike store (tri or not) and have the salespeople there put her on a number of bikes so that she knows how she fits on a number of brands. Ideally she'd get to try out the exact model of bikes she'd be interested in buying, but sometimes when you're doing beginning research you don't get that opportunity. But getting on bikes and seeing how they fit is really the only way to know what's best for a particular person.