As soon as Windows Mail is run, it gets all mail from the server, and empties the inbox on talktalk. But as you say there must be a setting to store on the server
PS Depending on how windows gets the mail, it most likely uses pop.talktalk.com or some such. if you change the pop it will not download mail and she will be able to check mail on the srver.
PS do not change SMTP as this is for sending outgoing mails