One thing that I was encouraged to do was get my hormone levels tested; birth control can affect a woman's testosterone levels (sounds weird, I know), and can cause them to go through the same motions as an older man who needs Viagra. If that's the case for her, there are a lot of different therapies and treatments to help get these levels back to normal.
Also, it's not recommended to be on birth control for over five years so if she has been, she should definitely look into other ways to curb that awful period.
I'd been with my boyfriend over three years when this started happening, we're rounding up on four now and working through it, luckily you seem to be as patient as he has been with me.