How do I deal with this?

So I’ve started dating this guy. And my best friend works with his baby mama. She told a friend at work that we were dating and now his ex knows. My boyfriend feels as if he was “robbed” of the chance of telling her and is really annoyed. I don’t know what to do because she’s my best friend but now I feel as if I can’t tell her anything and I don’t trust her anymore. What would you do in this situation?