Brian and Joe offer some good thoughts for you. To add to that, food for thought... if you are serious about offering her a co-founder status, you can create an agreement/plan for her to earn it. I've come across many startup founders that selected co-founders without the appropriate agreements. When it doesn't work out, they did not have a clear exit strategy in place. Very messy, and no one wins.
With regard to equity share, you can google business magazines, like Fortune and others, where their reporters did research and reported their findings on percentages of equity given across various job titles and with regard to the stage of the startup.
Also, consider using an assessment with her that will give you some measurement of her leadership and entrepreneurial abilities.
I find Joe's idea of collaborating with her on a job description, is a method I've seen work well. For your part of the collaboration, to be prepared, get on a job board where you can look up CFO, COO, etc, and VP roles. It will help you sort for the best title for her role, and also help you with selecting other KSAO's that will be best for your business.