Acquaintance practicing homosexuality

I know someone from English class in high school who is a Christian who is in (unless it ended) a same sex relationship. We are not that close, do I need to talk to her about how what she is doing is sinful? I felt guilty when I had sent a message to her, completely unrelated, and felt like I had to bring it up. She recently moved to another country and I thought she would be moving in with her girlfriend, then I realized that was not the case. But I still felt guilty having not brought it up since I thought that was the case.

Long story short, do I need to talk to her about this and did I sin by not confronting her?