My 12 year old dd is having a difficult time with a friend - not a school friend but one she sees on the bus every day and goes to Scouts with. This friend is being particularly vile to dd - and I have independent verification (!) that it's noting dd has done to this girl - it's just dd's turn to be her victim, if you see what I mean.
The problem is that her mother is a friend of mine - I see her a couple of times a week. If it was you, would you mention the situation with the girls to her? My inclination is not - but it's very hard to remain completely normal with the mother when I've just read a message on MSN to dd saying "Don't bother coming to Scouts tonight - nobody likes you because you're such a goody-goody. We have more fun without you"
Dd showed me the message, BTW - don't want to get into a discussion about snooping!