My girlfriend suddenly thought about her past

My girlfriend suddenly thought about her past. She also regreted that she should stop thinking about it. Because she is mature, she loves me a lot. So she doesn't want to think about it. But she yesterday just got memories back to her. I suggested her to open her past and tell me everything. She made hard to herself and told me. She was in her high school. She was not mature enogh, and she used to chat with her when she was 13-15 years old. Then when she turned into adult. She asked her to meet him. Then they met somewhere, and she confessed her feelings about her. He replied that he had feelings towards her before 5 years ago. He also added that he was in relationship with a girl. Though he told to my girlfriend that he still loved her. He also told to my girlfriend at that time that he can love a girl whenever he wants, he can hate a girl whenever he wants, he could control his feelings towards anyone. Myirlfriend shocked, and she asked him that how can he do like this? Then my girlfriend disconnected with him. But yesterday she told me that he was a nice person, though he breaked up with her, but he still loved me. She also told me that that person gave a biss happiness to her. Though i am in relationship with her since 3 years. But i couldn't give her that kind of happiness to her. I just want to ask you experties that my girlfriend is still impressed with this quality. Even my girlfriend is still impressed with his personality. I mean how can this kind of man to be a good person. Why my girlfriend still likes his personality. I see that that person who came into my girlfriend's life, he just used her. Because he did though he was in relationship with someone. If you are also agree with my statement about that person. Then how can i explain to my girlfriend that he was a wrong person for her. What should i do?