Ah.. the old "the prototype works so well, why do we need the real system?" That surely isn't an easy situation. No one wants to spend more money than they have to.
Would they be open to a conversation about the different goals of prototyping and why prototyping often looks like a complete system when it isn't? One analogy that you might want to play around with: a prototype is like the costumes and scenery in a period drama. The costumes often look authentic and they successfully support the story. You can dance and sing and act in them.
However, those costumes are rarely wearable on the street. They would never make it through the washing machine. Seams are basted or pinned rather than stitched. Materials that look like silk are used instead of real silk. Cloth with the durability of gauze is used instead of sturdy broadcloth. Buttons and zippers are fake - up close the clothes use cheap snaps and clasps that easily tear off.
A prototype is like the period costumes. They are good enough to give a realistic feel for what the system does and to ferret out any logical inconsistencies in the design specs. However, they aren't designed for operational strength because that would take time and attention from the more important job of making sure developer and client truly understand each other. If you want to wear stage costumes on the street you have to turn those pinned seams into stitches. In the same way if you want long term durability and security, you sometimes have to rewrite parts of the prototype in a more robust medium.