I would remove her account from outlook express. Click tools -> accounts -> mail ->choose account and remove.
You should be able to add her account to outlook if it's not already there in pretty much the same way it's removed from outlook express.
Make a shortcut to outlook named "hers" and make a shortcut to outlook express named "his".
To change the default client, open internet explorer and click tools -> internet options -> programs and choose the one you want to be default