Not-so Platonic friend tells my girl friend he loves her and she said it back. Should I be worried?

My girlfriend and I have been dating for about a year now. Yesterday, her platonic friend told her that he loved her knowing that I was sitting next to her. She replied it back. I normally don't become insecure in these situations but she has admitted to me that this guy has already tried to to hit on her in the past. Her reasoning for saying "I love you" was that she has known him for a long time. Before we started dating, we were close friends for nearly 10 yrs and she has never said those words to me even as friends. I trust her as a girl friend but this whole story sounds too shady and I can't really make sense of it. I just need some perspective on the situation. Should I be alarmed? Is there something she isn't telling me?