The Flirty Text That Broke Jake

AITA for accusing my best friend of cheating on her boyfriend after I caught her in a lie? I never thought I would be in a situation like this but here I am. My best friend Sarah and I have been close for years. We tell each other everything. Or at least I thought we did. Recently, she started acting strange around me. She would cancel plans last minute and avoid eye contact. I asked her what was wrong but she always brushed it off. Then one day, I saw her texting someone late at night. I did not think much of it until I noticed her phone was unlocked and I saw a message that made my blood run cold. It was from a guy named Mark. The message was flirty and intimate. I confronted her about it the next day. She looked guilty but denied everything. She said she was just friends with Mark and it was nothing. I was furious. I could not believe she was lying to me. I decided to dig deeper. I looked through her phone and found out she had been talking to Mark for weeks. Then I found out she was actually seeing Mark behind her boyfriend Jake's back. I was stunned. I never liked Jake but I never thought Sarah would cheat on him. I decided to tell Jake everything. I sent him screenshots of the messages and begged him to believe me. He was devastated. He confronted Sarah and she broke down. She confessed she was bored with Jake and felt a spark with Mark. She said she never wanted to hurt anyone but she could not resist the thrill. I felt betrayed but also angry. I thought Sarah was my best friend. I thought she was loyal. I never expected her to be capable of cheating so blatantly. Now Jake is heartbroken and Sarah is trying to apologize but I am not sure if I should forgive her. Did I do the right thing by exposing her or did I ruin everything over a mistake? I am still reeling from this mess and I wonder if I should have kept her secret to protect her. Or am I justified in revealing the truth? I do not know anymore.