Help! Is it normal that... I'm keeping a secret from my friend about her daughter?

This one’s tough. Can the ivillage brains’ trust help this mum through another difficult situation?

My daughter’s 13. She has a best friend who I really like and I’ve got to know her mum. We all do things together and spend quite a lot of time around at each other’s houses.

I didn’t know this mum until our daughters became friends at High School, but now I’d say we are quite close. Sometimes our girls go around to each other’s house after school when one of us can’t get out of work. We’ve enjoyed a few wines together and see each other at Saturday morning sport.

My daughter and I are very close. It’s just the two of us and I try to be open with her so that she can feel she can come to me with her problems. I was a terrible teenager, doing all sorts of things behind my parents’ back and so I’m realistic about what teenagers get up to, but I’ve always wanted to make my girl feel she can tell me what’s going on.

She tells me that her best friend has starting smoking pot with a boy she’s seeing secretly. My friend, her mother, has no idea.

My daughter is desperate for me not to tell anyone. Now I feel terrible. If I was my friend I would definitely want to know, but there are two problems – breaking my daughter’s trust, which makes it less likely she’ll tell me what’s going on in her circle again, and also if I tell, my daughter will lose her best friend, who will be feel furious and betrayed. I know I’m the adult here, but I really feel torn. What do I do?

What do you think? Should this mum come clean with her friend about what her daughter’s up to? Or is protecting the trust her daughter has in her more important?