Desktop Redirector doesn't support groupwise therefore assuming no BES only option is to setup BIS and integrate work email via pop3 / IMAP or forward mail from the mail server to external account.
With BIS you can set the reply-too address to
so that if she sends an email from the BB then it looks like it has come from hercompany.com, rather than from her BB account e.g.
The only way for her to get copies of her response back to the main mailbox is for her to BCC herself in. - which off coure then loops back around to her BIS account, unless she bcc's a 3rd account.
basically non BES it's pretty lame cf. BES