I think its awesome!!! Yes there's sugar in fruits, but its better than eating refined sugars!! There are so many health benefits in fruits..vitamin c, antioxidants, etc...;-)
as long as she is eating a balanced diet its fine. at the beginnng I loved orange juice too actually any juice. I have gotten into the habit of watering it down because I was warned that if you only drink juice it can cause gestational diabetes issues later on. but for the most part it is perfectly fine.
As the previous poster said, as long as she is eating a balanced diet otherwise (and taking her prenatal), it's fine. If she has been feeling sick, she may find fruit easier to eat than many vegetables - for me, I could stomach citrus fruits and a few others but most vegetables made me sick. So, I ate a lot of fruit to compensate fiber-wise for the lack of vegetables. I went back to veggies after my nausea subsided.
It's funny how everyone seems to prefer citrus flavors!