What about using the Qt framework(Qt | Cross-platform software development for embedded & desktop[^])? You could build the application on Windows, iOS, and Android. If you share your final source code with your customer, you may be able to use the Open Source version, but I would check the licensing restrictions. You could also check out Felgo(Felgo: Build Native Cross-Platform Apps and Games. Rapidly.[^]), as well. It is build on top of the Qt framework, but their licensing seems more understandable. Cheers!