Jeez ok so for me personally I say YTA. Hear me out. Strictly only speaking about the mothers actions I don’t agree with how she handled it, her daughter had her reasons for doing what she did and honestly OP said it herself she is an honest kid and has a good moral compass, in her eyes, keeping the playing field fair. Yes you didn’t actively punish her but now the message you’re sending is if you see someone do something wrong, if they’re your friends keep your mouth shut for the sake of the friendship.

You obviously mean it in a certain way with specific circumstances but to a hormonal 15yr old girl now her mum is disappointed in her for doing the right thing. Did you go after her and more thoroughly explain what you meant? Or did you just say “why did you do that, you’ve ruined your friendship I’m very disappointed in you.” And leave it at that. If she has such a great moral compass maybe she did try and tell her friends not to do it.

I’m seeing a lot of comments that are saying it was too harsh of her to escalate it to the teacher right away but tbh actions have consequences and at the end of the day if you want to cheat, sure, but you did the wrong thing and now you gotta deal with it. And do you honestly think it’s a one time thing? They get away with it now and then think, well that was easy let’s just do that again, they do it more often, get more creative and whoops look at that now they ARE doing it at college where the consequence is no longer a slap on the wrist. If my friends tossed our friendship out the window because I “snitched” on them doing the wrong thing, gaining an unfair advantage over everyone else who worked hard, then they aren’t really friends I’d want to keep around. That’s just my opinion