How to get the value of a 25percent ownership?

I am 50 %owner of a small s-corp that started in 2009. My business partner owns 25% and his wife owns the other 25%. She is not active in the company and never has been. They are currently in the process of a divorce.I have been notified by her lawyer to provide the value of the 25% ownership. How do I go about that? I use Quickbooks 2014 for the accounting and it shows the net worth of the company but I have not entered any depreciation for this year ,still waiting on accountant to guide me through that process. Any help would be greatly appreciated.