Caught best friend with my ex!

I (34f) recently discovered my best friend (35f) is dating my ex (38m), whom I broke up with after a serious relationship. She didn't tell me directly; I found out from mutual friends. Feeling hurt and betrayed as my best friend knew all about our past. It's affecting our friendship, and I'm confused about how to handle this situation. Should I confront her or just let it be? Any advice? #WhatWouldYouDo #FriendshipDrama #TrustIssues