This couple isn't in Downriver so thankfully we aren't talking about the same people
My personal feeling is that she needed to know (not that I wanted to be the one to tell her but still). She had suspicions, but nothing concrete. He's been trying to guilt trip her into thinking that she was "bad" and "in the wrong" all the while still trying to get her into bed! One week Tuesday he's like that, Wednesday changes the locks, Friday is with this new girl. I think he's just upset that he got caught.
All the "Don't tell" crap that went around is just that, crap. If no one was supposed to know, then no one would have said anything. Could you imagine if she went into this blindly?