The fact that she's always had access to her boss's email makes me wonder what the fuss is about - did he not know you had access? You might have an opportunity to show that you are proactive and responsible by suggesting to him that you contact IT yourself to remove those folders from your Outlook set up. If he needs you to action something in future, he can send it to you.
Also, just a word of caution - if you have access to his emails, maybe he also has access to yours, or other people do (other than IT of course). Best to keep your nose clean at work and not use it for anything personal (including online shopping etc).