My Dilemma at Lunch Today

Today during lunch, I couldn't help but overhear my friends discussing a project they're working on for our history class. They seemed to be making good progress and were talking about how they'd split the work evenly among themselves. But then it hit me; one of them had accidentally skipped a section that's crucial for understanding the whole assignment. I know this could potentially help us get a better grade, but revealing what I heard feels like betraying my friend’s trust. Should I say something or stay silent? What would you do in my situation?