Hmm. Rules are an Outlook thing, so you might need to post in
m.p.outlook for the most expert answer -
Is this rule a client- or server-side rule? I'd assume it's a
client-side one and won't do anything unless she's logged into Outlook
at the time.
Try setting up another autoreply rule (use a test mailbox and set up
one that will autoreply to all incoming mail).
That said, I think enabling autoreply is a bad idea as it can lead to
mail loops.