finbar – if she is doing triathlons then surely she will be in an aero tuck most of the time, so that’s exactly what she wants. Why should she buy a bike which is better for some other theoretical thing which she won’t actually be doing?
Because riding in an aero tuck – properly – is bloody hard work. It requires a lot of flexibility, different muscle strengths to regular cycling position (the neck and back can get particularly sore), and unless you’re only ever riding on pancake flat roads it will probably be less efficient than a regular road position anyway. Tri bikes just aren’t that suitable for beginners.
Also if someone only has one bike, a road one (perhaps with clip on aerobars) would be much more versatile – most triathletes will keep their tri bikes for races and key workouts only, and ride a regular road bike the rest of the time.