If your in the Uk, the best option is probably going to be ordering online to avoid any issues and get the best discount.
I believe the Uni specific Stores and the NUS store have higher discounts then instore, at least from what I found when I was checking.
She probably has her NUS Extra card, so the easy option would be to log into the NUS Extra Site and follow the Link to the Apple HE store from there.
That site can be accessed from anywhere, and once your there, they don't ask for any other confirmation. I recently ordered My Macbook through that link, and was not asked for any proof of student ID throughout the process.
If your Unable to Accces it, PM me and I'll Shoot the link to you.