I would also add the modifier of why he wants to break up. If it’s simply because he doesn’t see marriage/a future together but still cares about her it’s a very compassionate thing to do. If OP is unhappy being with her, than I’d say it’s important to find happiness. As long as OP is still happy in the current situation than I’m totally with you. Most importantly though, as you said, if she comes up and says that there’s obviously something wrong or something of that nature, it’s time to just be honest because letting them sit there and try to figure out what’s wrong is more cruel than just telling them imo