As the person himself (or herself) what happened (and ideally what they learned from it). If they are an honest person they should tell you.
I was engaged to someone else a few years before I met my wife, and it was broken off. While I don’t talk about it (or even think about it ) very often if someone asks me about it I will give a brief explanation of what happened and why. And before anyone asks my Wife does know about it and did before we got married.
The very short version is we realized that our lives were going in very different directions.