Tools | Accounts
Double click the account.
On the Advanced tab check the box to lkeave a copy on the server AND the box
to delete after 5 (or whastever) days.

To share a common OE store between different WinXP User Profiles on one
machine, see http://kellys-korner-xp.com/xp_oe.htm#xp_share_address.

Sharing a common store on two machines across a network, while possible, is
exceedingly difficult and fraught with peril (e.g., loss of all messages in
the store).  See http://www.insideoe.com/faqs/why.htm#network.

She could use MVP Steve Cochran's OEX (US$19.95, currently) to import
messages from one machine's, e.g., Sent Items folder to the same folder
(i.e., Sent Items) on the other machine, thereby keeping them in sync.  NB:
Doing so will overwrite the content of the folder to which the messages are
imported!

It's easiest enough to keep each Inbox more-or-less in sync w/out resorting
to same.

On both machines:

   Tools > Accounts > Mail > [mail account] > Properties > Advanced >
Delivery > Leave a copy of messages on the server (check)

   Tools > Accounts > Mail > [mail account] > Properties > Advanced >
Delivery > Remove from the server when deleted from 'Deleted Items' (check
if or as desired)

On one machine:

   Tools > Accounts > Mail > [mail account] > Properties > Advanced >
Delivery > Delete messages from the server after XX days.