If she was at somebody elses house and told Aim to remember her password, then this is how this happens.
Just change the password and do NOT tell it to remember your password. This will stop this from happening
Last night my 2 daughters and I were on our pc's and noticed that my youngest daughter's old screen name was in use with an away message. We IM'ed the screen name with no anser. No idea how somone got the password, but my daughter changed it. How can I report it to AIM services or should I not bother?

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