It’s the same app, regardless of the user location. When it’s added to the existing system it acquires data from that system.

If parts of the app are greyed out it’s most likely to be due to a version mismatch. The downloaded app would be the latest version; the system is probably weeks if not months out of date. In that case the system just needs an update, for which of course an internet connection is required.

If the new tablet with the new Sonos app doesn’t allow the customer to connect to the system, how can he update?

The app should always allow initial connection. You said it connected but wouldn’t allow changes, which strongly suggests a version mismatch. Presumably the system’s been at sea for a good while so its firmware is almost inevitably out of date.

The user has installed the correct app: S1 vs S2?

The customer tells me that the devices don’t require updating.
The client wanted to add amazon music to the sonos system but that didn't allow it.
Do you have any opinion?
We had to reset all the amps.

I doubt that you needed to reset anything, if that’s a factory reset. It’s hardly ever the solution.

How do you mean that the system wouldn’t allow Amazon Music to be added? If it’s not even on the service list then the country on the user’s Sonos account is set incorrectly. If it’s on the list but throws an error when attempting to add it there could be something wrong with the Amazon account. Without a screenshot it’s hard to say.