Is my best friend really "my best friend"

Sierra • |God ✝️| Dustin❣️| beaches 🌺🐢| Libra♎️|
Okay so I found out that my best friend smoked weed with my ex.She knows that he hurt me bad and she says she can't stand him anyways. I was hanging out with her and he texted her and I was like what was that and she said nothing. Now she says she is going to the movies with her friend in Lakeland and my ex happeneds to live there. So I'm conflicted and I just need help if I should still be her friend we have been best friends for 12 years....or should I just stop talking to her and not be her friend. Please help. ASAP!