I got caught up in this situation some time ago. A girl and I dated briefly but nothing too serious happened. She started dating a guy and they got serious. They moved in together after 6 months. Lasted two years. After a trip abroad (a trip that apparently didn’t go well for them) she contacted me and we started hanging out again. I really don’t know what she wanted (to see if there was a spark, have some fun or just wanted to chat?). But either way, I didn’t pursue because I’m a good guy but I think she felt rejected. A few months later he walked out on her. Later I initiated contact but to no avail. Live and learn - everything has repercussions.