I need advice!

Me and my boyfriend recently broke up (we ended on good terms) and today I saw him at lunch with this girl I’ll call Sarah. Sarah had a huge crush on him before we started dating, and openly let me know that he was out of my league. She would ask mutual friends of mine and my ex’s saying things like “she’s not right for him” and “when are they going to break up”. In relationships she’s a nightmare. I want to protect my ex, as we’re still close friends, but I don’t want him to think I’m jealous! If it were any other girl I would be so happy for him, but he has no idea what kind of person ‘Sarah’ actually is. What should I do?