Is she a "authorized user - retail", she can then upgrade.
However, keep in mind that it will automatically delivered to the billing address that is listed on that account, there is no option to choose which address to ship. So in this case, it will most likely be delivered to her parents house. If that's the case, either she needs to be at her parents home when it's delivered or have them be informed that there is a package.
My mom is the main account holder, while I'm the authorized user. I can pretty much do anything like pre-order, and make changes to the account (i.e changing plans).
The above method may work if she can log into her parents account online (may need permission).