If the client is demanding a cross platform app using Flutter which looks iOS on iPhone and Android on Android devices then you have to make it clear to the client that it is not advisable and may end up a lot of work and hard to maintain in the future.

If the client does not agree then charge them more for this feature.

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store