I'm reading the responses here aghast
it's like some of you have never been fourteen before?
If another parent stepped in because my DD couldn't come to me herself I'd be 1) appalled at my own parenting skills and disappointed that our relationship was such that she didn't feel she could trust me, and 2) extremely grateful that the mum stepped in and helped.
The people who would be fuming that another parent had the good sense to get your daughter the help she didn't feel she could come to you for? Seriously?
Op you cannot break your daughters trust like this by telling her mum or calling the school. If you do, I guarantee you'll regret it later down the line when your DD doesn't come to you.
I was in a similar situation myself at fourteen and my strict mum found out (she went through my private chats). She told the boys parents, so he told his friends, who then told the whole school. I was the girl who's mum found out. Then she grounded me for three whole months. I missed birthday parties, the christmas disco. Everyone at school knew why I was grounded, it was like a daily reminder of me being a slut, with all the slut-shaming that goes with it. During that time all the friendship groups at school shifted and by the time I was "ungrounded" I had lost so much confidence and was honestly spiralling into depression.
Sounds dramatic now at 30, but when your 14 everything IS dramatic.
My relationship with my mum NEVER recovered. There were things far more "dangerous" as our friendship group got older and I remember my best friend being able to tell her mum everything -- underage drinking, date rape, drugs, an abusive boyfriend, etc. I couldn't talk to my mum about any of this or the things my friends were doing because I couldn't trust her not to break secrecy.
Don't be that mum.