My best friend's boyfriend is cheating on her?

So yesterday I was planning on getting chipotle with my friend and her boyfriend. I always call the location to ask how long the line is before we go just to see if I should order online first. My phone was nowhere to be found so I asked to borrow his. I asked Siri to call chipotle, which usually calls the nearest chipotle. Chipotle was in his contacts. Suspicious. I called it and a girl answered saying she wasn't chipotle and cried when she realized she was a side chick. I hung up and confronted him in private. He gave me $50 to not tell. I took it but of course I'm going to tell. But I'm trying to figure out how. I feel like "your boyfriend it cheating on you." Is too harsh. Should I tell her the story like I told it on here?