The primary difference between men's and women's road bikes is that women's bikes have a shorter top tube in relation to the length of the seat tube. The design accounts for the fact that women typically have longer legs and shorter torsos than men of the same height. It used to be that women would ride shorter stems to achieve the proper position on men's bikes, but this compromised the handling characteristics. Thus, women specific geometry bikes were designed.
If a woman has a long torso, she may likely fit better on a men's bike. If a man has proportionally long legs, he might fit better on a women's bike (a tough sell for your local shop).
If you are curious about the fit of your dad's bike, take it to the best fit specialist in your town and have him assess your position.