Help Fiance 23M and bestie 21F

I am 23F and my fiancé of 7 years is 23M. We have had our ups and downs but have been together since teenagers. We now have our own house with lots of animals. My fiancé has been sober from alcohol for one year. When we were in our 20s we moved onto the property we currently live on, my fiancés dad owns the property and is incredibly narcissistic. This deeply affects my fiance and he ended up learning he has a problem with alcohol. Life has gotten significantly better since he has stopped drinking. I have an absolute bestfriend of 3 years (21F) I have been through so much with this girl, I even sat by her bedside while she gave birth. Recently she has gotten out of a relationship with her baby daddy and has completely changed. My fiancé and I were fighting this weekend, and he informs me that he did coke with my bestfriend 6 times when he was drinking. They all must’ve done it within a crowd of people, but they would do it every Friday when I had to work. At the time he did tell me he was doing coke, but I’m just finding out a year and a half later. He said he never told me because he didn’t want me to loose the only friend I had but she’s gotten really toxic and thought it was time to tell me. I’m so hurt because I would come crying to her over him drinking and doing drugs, and she would go behind my back and do coke with him? And never once has mentioned it. They both agreed to not tell me apparently. And it’s so sad because if she would have told me at any point in the last year and a half I would’ve completely forgiven her.