Think bank account numbers and investment information etc. He must then also be able to update certain information like his adres
1. Storing financial information on a server requires advanced security measures.
2. Storing a customer's personal information shared in the registration process like name and address are not as stringent.
3. There are plenty of registration or member plugins than can do the registration process (name, address), but you need to review storing and using financial information for clients online for their protection and yours (use a third party secure application) and certainly SSL.