It sounds like the start-up is in need of a business analyst (BA). And this could well be the role of your accountant friend. I am an experienced business analyst with a technical background, although I know many business analysts who have little or no technical background. The role of the business analyst is to work with the stakeholders (e.g. developers, users, management, etc) to design solutions (technical or not). The business analyst creates documentation (user stories, business requirements, business logic flow diagrams, etc) by working with the stakeholders. The developers and testers then use this documentation to develop the solution. There are many business analysis books out there, one of the most popular is the BABOK (Business Analyst Body of Knowledge), see https://www.iiba.org/babok-gui.... It has many tools that a BA requires. But I don't recommend your friend becomes a full blown BA, but it may help to learn some tools and techniques described in the BABOK.
I always see the Business Analyst as an interpreter or go-between, between the business and the developers. And the Business Analyst uses tools (i.e. methods of documentation) to formally describe what the customers want.