She can if she wants. If she's not familiar with vegan diets, it could be a good idea.The intake of protein, fats and other micros could become a problem. Remember a healthy diet is first, balanced and second, a diet you can stick to.
Wouldnt talking to a dietitian help or even be a good idea for anyone considering something like this? Or is that not in line with what a dietician would normally advise on?