My child logged in IM @ friends pc. How do we stop friend from using ID now?

This child's parents say that @ time they log on now yahoo is their homepage and my child's IM is automatically logged in. I want to walk her thru deleting my child's id from her pc completely. There have been some violations - using profanity and etc. contacting people on my child's list, by this other child and I want out of this nightmare. Please HELP!!!