You definitely should talk to an attorney.
If the business has been "in business' for only 4 months, it does not seem reasonable for her to take a chunk of your profits in future years.

You say "Our business is really not established in any way at this point - we have no product, no customers, basically nothing." so maybe your best bet would be to dissolve this business.

You could probably start a different, new business on your own more cost-effectively than buying her out. From what you say, what's there to buy?