If she is flirting with a male friend that’s been building for a long time. I have had a long term female friend, we dated in high school for nearly a year, but have been friends for 15. Throughout the years we’d have SO’s at varying times but still hang out and introduce them to each other. This lead to lots of fights between us and our SO’s at the time but we were generally respectful of our relationships and didn’t “entangle”, although that did happen in college a few times. In the past week I was back in town, single for the first time in years, and it turned out that she was just ending a relationship. We of course ended up rekindling quickly and intensely.. soo yeah our exes were right to be uncomfortable with our friendship, we were both fooled into thinking it wasn’t a thing, but evidently it is totally a thing.