I think the issue comes that it only syncs messages one way and doesn't sync on deletions. Apple should have messages that are removed from all devices when removed from the phone, but it didn't remove messages when deleted.
Sure the guy is a moron for being a cheater and scumbag, but Apple should remove deleted messages. That's a privacy problem with Apple's sync. I don't use Apple devices due to other Apple crap, but setting up iCloud sync should have a warning when items won't be deleted and only will be downloaded to devices.
Wasn't an entire stupid movie about the horrible sync pitfalls in Apple devices premiered years ago?