this used to be me. constantly sick with sinus infection (like out of action for ~10 days every 2 months). I started dating someone who ate a vegan diet, and after adopting that diet I've been sick twice in 4 years and both times were very light and ran for only 2-3 days.

I have no idea why it had that effect for me, and everyone is different, but if she's suffering enough that she's at the point where she'll try anything you might pass along my anecdote. I wish I had chanced into it earlier.