Hello Rob Broadbent,
If you've already turned off Messages in iCloud in Settings > [name] > iCloud > Manage Storage > Messages, the messages should be downloaded to the device, and should be present in the Messages app. If they aren't visible in the Message app, you'll want to tap "Undo Disable & Delete" in order to get your wife's Messages downloaded.
If necessary, Messages in iCloud is enabled, visit Settings > [name] > iCloud and turn off Messages here. This will disable Messages in iCloud for the device and download her messages to the device. Once the data is visible in Messages, it should be safe to then navigate to Settings > [name] > iCloud > Manage Storage > Messages and tap "Disable & Delete."
Let us know if this helps!
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