All he had to do was put his wife on a different account on the Mac or use another messenger on his phone. I don’t see iMessage as being “leaky” in this instance. His messages didn’t appear anywhere they weren’t supposed to from a technical perspective. He used the same account on the Mac and iPhone, syncing messages worked as advertised. I’d expect this to happen with any message sync feature, it’s not iMessage specific.
It’s like complaining that your wife found out your were cheating because you used FB messenger, yet didn’t create a separate login for your wife on your Linux desktop, and the sole account’s web browser is logged in to your Facebook. He fucked up, that’s poor computer security to let someone else use your account. A major Mac feature is a lot of activity is easily shared across devices you’re logged into. Photos, messages, calendar, reminders, all sorts of things. This tells me to be careful where I log in with my iCloud account and who uses it. Why would you not have a separate login for your wife, especially if you’re fucking around on her and she regularly uses that computer?