When I attempt to delete her account on her Mac, I get a message, which is not mentioned in the link, that says: 'Are your sure....deleting this account will remove its data from Contacts, Mail, Messages and Notes.'
You should only lose everything if your account is POP. If it's IMAP these emails etc. should be on the server. You could log in to web mail and check all the emails are there. If you're not happy deleting the account then uncheck the box that says 'Enable this account' in the Preferences. The account will be greyed out on the list but will still be there.