Relationship boundaries vs. Friendship boundaries

(Throwaway account for privacy) I need advice on what you and your partners keep private vs. what is okay to share or discuss with friends, etc. My partner tends to overshare anyway but just told her friend things I considered confidential between us. When confronted, she deflected and got mad at me, saying no one cares about that anyway and I should relax. I think she's missing the point, that now I don't feel safe telling her anything.