Lou's Yahoo Mail account got hacked and has been used to send out spam. He's changed all his passwords, but he's worried that they now have control of his iPhone. Leo says that didn't happen and Lou is being understandibly paranoid about it.
Leo advises changing the password and then turning on two factor authentication. This is usually done by giving Yahoo his phone number and then they will text him if his password is being changed. He'll input the code and then the password gets changed. This prohibits someone from changing the password unless they steal his phone first.
The "UPDATE Protocol" detailed in the Future Crimes Book is a good way to stay secure online.