Backdoor Question

First off I am not sure I am posting in the right place. A friend of mine asked me if it is possible to get backdoor access thru yahoo IM. They had received an offline message from someone they did not know. They answered The IM when they got online saying they didnt know them and blocked the sender.

A few days after the IM some of their paid accounts at different sites said they no longer had access. According to them this is the only possibility of them losing the accounts. All the accounts they have lost keeps telling them they have the wrong password.


Thank You