AITA for exposing my best friend's secret boyfriend?

AITA for exposing my best friend's secret boyfriend? I never thought I would do something like this, but I am so furious right now. My best friend Sarah and I have been inseparable for years. She is the sweetest person ever, and I always wanted her to find happiness. Last year, she started dating a guy named Mark. He seemed perfect at first. Charming, funny, and attentive. But I had my suspicions from the start. One night, I accidentally saw Mark's phone light up with a message from a girl named Lisa. The message was flirtatious and very suggestive. I was shocked. I confronted Sarah about it, and she brushed it off, saying Lisa was just an old friend. But I could tell she was uncomfortable. I decided to dig deeper. I secretly installed a spy app on Mark's phone when he was over at my house. I wanted to see if he was hiding anything. What I found was worse than I imagined. Mark was messaging Lisa daily. Their conversations were full of innuendos and plans to meet up. I was about to confront Sarah again when I discovered something even more shocking. Mark was cheating on Sarah with Lisa. The two of them were supposed to be just friends, but they had been secretly seeing each other behind Sarah's back for months. I felt torn. I loved Sarah so much but also felt betrayed that she had been blind to his lies. I debated whether to tell her or keep quiet. But I could not stand to see her hurt anymore. One weekend, I gathered proof. Screenshots of the messages, photos of their meetups. I knew I had to tell her the truth. I called Sarah over for coffee and showed her everything. Her face turned pale as she read the messages. She looked shattered. She asked why I had done this. I told her I could not watch her be deceived. She started crying and said she had loved Mark so much. She felt so betrayed. I thought she would be angry at me, but she just looked at me with tears in her eyes and said she needed time to process. The next day, Sarah confronted Mark. She told him she knew everything. His face turned red with shame. He begged her for forgiveness, but she was done. She ended things immediately. I thought that was the end of it, but then I received a message from Mark. He claimed I had invaded his privacy and that I was the real villain in all of this. Now Sarah has not spoken to me since. She says I betrayed her trust even more by spying. I feel like I did the right thing, but I also never thought I would lose my best friend over this. I am not sure if I was the one in the wrong. Am I the asshole for exposing Mark and causing all this chaos or was I just protecting Sarah from a liar?