Hi Daniel,
I see that there are only few users (12) and customer does not have SAP implemented; nor the customer is planning to have any of the ERP Modules, which makes it a costly solution if they want to go for the only solution related to this.
If you want to setup Fiori or WebDynpro, you would need the following as a high level requirements:
1. Fiori ->a DB, SAP ERP, NW Gateway, UI5 Addons, proxy servers etc..
2. WebDynpro-> a DB, SAP ERP and develop your customer WebDynpro Screens
Additionally there are many things to address security like SSO, X509; (Active Directory) etc... I would say if the requirement is small for 12 users SAP would not be best fit.
If you are looking for a huge user base with great advanced features like Responsive UI screens which can work on Desktop, Tables etc... Fiori/Custom UI5 developments would be best options instead of WebDynpro.
You can find more details on Architecting an SAP Fiori deployment
SAP Gateway deployment options in a nutshell
Alternative way is to develop your Custom solution which can communicate with your existing SQL Server and develop your own responsive web apps or Hybrid apps which can work quite good without any compromise on security.
Hope this helps, if any further questions please ask.
Regards,
Nagesh