Not an iPod nano, no. It's impossible to check e-mail on an iPod nano. However, it is possible with an iPod touch if they added their account to the Mail app or had Safari store their login information. However, checking one's e-mail with Safari and not storing the information means that you are safe. You should change your e-mail password if you are worried. Changing it on a PC means that any other device (including an iPod touch) with a stored password will require the new password before accessing the e-mail.