Should I tell her mother?

[mikedec5to14]

Dont betay your 12 year old friend. Find a time when the three of you are together (so the 12 year old can know exactly what6 was said) steer the conversation gently, and make a very casual remark about how its a good idea for parents to monitor what their kids do online. Try to make eye contact with the mother as you make the comment. If she is sharp, she will pick up on it immediately.

If you are convinced she did not get the message then go over to her privately later and just say "that comment i made about monitoring twitter accounts was important".

Then step out of it. If mom does nothing, then you simply forget about it. You will have done what you agreed to do.