Back the fuck off. You don’t need to warn them, press them about it, or nag at them. Leave them alone. People are different with every relationship they’re in. You can’t accuse someone of being a bad girlfriend or boyfriend if you’re not even in the relationship. People do change. I sure as fuck wasn’t the same person I am now that I was a year ago. People grow up. People settle down. But with all your noses in their business, it’ll just ruin what they have. Learn when to keep your mouth shut.