it doesn't appear as though there's any "on read" trigger for a rule, and it doesn't look like "read status" is a rule criterion that is allowed either.
You can probably do this with an Applescript, but then she'd have to run the applescript when she wanted read mail moved anyway.
It can't be that hard for her to drag read messages into and "old mail" folder, can it? (especially once you show her how to do multiple selections with the shift and command keys?)