AITA for wanting to break 'guy code

Okay, I need your judgment. My best friend's boyfriend Mark is part of our larger friend group. Last weekend, a bunch of us were at a party, and I overheard Mark on the phone in another room having a very flirty, obviously inappropriate conversation with another woman. He wasn't just being friendly; it was full-on emotional affair territory.

My best friend (F36) is head-over-heels for this guy and thinks he's perfect. Part of me thinks I need to tell her immediately. The other part of me knows this will blow up our entire friend group, and I'll be labeled the drama-starter. I also feel like there's an unspoken 'rule' to not get involved in other people's relationships. So, AITA for wanting to tell her and potentially destroying the peace?"