imessage in CHina

https://www.apple.com/privacy/features/

Your Messages and FaceTime conversations are encrypted end-to-end, so they can’t be read while they’re sent between devices.

End-to-end encryption

End-to-end encryption protects your iMessage and FaceTime conversations across all your devices. With watchOS, iOS, and iPadOS, your messages are encrypted on your device so they can’t be accessed without your passcode. iMessage and FaceTime are designed so that there’s no way for Apple to read your messages when they’re in transit between devices. You can choose to automatically delete your messages from your device after 30 days or a year or keep them on your device indefinitely.

iMessage apps

iMessage apps — which let you share stickers, songs, and more without leaving Messages — do not have access to participants’ actual contact information or conversations. iOS and iPadOS provide each app with a random identifier for each participant, which is reset when the app is uninstalled.

If her Apple ID is registered in the US, her data will be stored in US servers. iMessage for Apple ID's in the US should be secure.

If you use iCloud to backup and you are a Chinese citizen, your data is stored on servers in China and could be accessed by the government.

This is true of any country.

Apple has to comply with the Law of any country they do business in.