I think My best friend likes this guy more than me And I think shes cheating on her boyfriend!?

My friend Lucy has a boyfriend. But anyways one day I was talking to Joe (Lucys boyfriend) he said he had a new girlfriend (this was before I knew Lucy and Joe were dating.) So I was joking around saying, "I will ask every girl in the school then!" And he said that he told his GF not to tell anyone. So i rushed over to Lucy and asked. She said Joe told her not to tell anyone, so I figured it out. Then when i was all smiles and laughing at her as a friend. Then she just told Micheal (A kid who really likes her) and I turned to frown. I walked over to her and said, Why did you just tell micheal, when you made me figure it out? but she just shrugged.Then when we went on a Field trip, I wanted Lucy to sit with me on the way back to school. But instead she sat between Micheal and this kid Harold. Then we all realized (me and my friends) that Lucy and Micheal were flirting. (Joe didnt want to go on the Field trip so he stayed home) Micheal gave her a flower, and Lucy leaned on his shoulder. Do you think she's cheating on Joe?