Access Custom Membership Provider from Outside Apps

Our custom membership provider was written by an outside consultant.  

I am trying to figure out how I can change SF user info from my Member Management System which is a dot.net project using SQL for data storage.

Our application that stores our SF ID,  SF username, password, salt, isApproved in a SQL table in the Member Management System, from the code I can see that this data is created in SF first. 

I have tried to copy the our SF dll into the project and I can't get the objects to instantiate when I debug the code.   I wrapped it in a Web-Api project and I was getting the same issues.

Thanks JC