Many people believe that observable patterns provided the foundation for astrological predictions, but I think it was originally a way to encode arcane knowledge in a more "hip and fun," easily-remembered myth structure.
She's right, though, that it can't be proven right or wrong, just like any other myth or belief.
We simply don't have the data. Remember that actual proof must follow the guidelines of the scientific method and not just include personal experience or anecdotal evidence.
Most divination is intentionally vague nowadays, anyway, to allow for personal interpretation, so perhaps she just finds it a useful tool for soul-searching.
On the other hand, perhaps she spends a huge amount of time doing the math to draw up specific astrological charts (or uses the software!) and gets answers that anecdotally apply to her experiences.
The fact is, this is not a battle anyone can win, captain. It's just what works for her.