This is not Apple’s fault. I have all Apple products and my messages only come on my phone. There are settings to stop a sync of messages between the two devices and you are asked at set up of either device and you have to confirm it. To many people skip all the set up reading and just hit continue.
You guys should be the ones being diligent. I get this story is a little different in that he thought deleting the conversation happens on both but why didn’t the OP check. And double check. And triple check???? If you knew that messages sync - why would you even use the iPhone to book to begin with?
This is your life guys. You are responsible for keeping your hobby from your wife. Not Apple.
OP - I do feel bad for you but if you are going into any conversation with wife with the feeling this apples fault and not yours - you are certainly headed for divorce. Women can tell when you are only saying you are taking responsibility and not actually doing it. You want to save your marriage / take the responsibility, so the right thing, be honest, beg her forgiveness, sync all your devices so she can see everything and keep it your pants. No more hobby for you.
or get divorced. Read some threads. Some guys would dread it and then when it happened, they say it was the most freeing thing in the world.
Maybe set up a session or two with e a therapist to talk out what YOU want going forward in your life. I mean there is a reason your started hobbying to begin with right. Maybe look at that.
just some suggestions. Good luck.