No, you basically can not do that, at least not as simple as you described it.
Visiting air-side transit areas is usually allowed for transit passengers who have no visa, hence the name.
I am not aware of situations where people who live in a country are allowed to visit there. One of the reasons is exactly what you are trying to do, duty free goods.
Even employees who work in such areas, will undergo security checks upon exiting, they will be checked for any stuff they brought from inside, including duty free items which they are not allowed to buy. Your case is similar, who will pay the tax for the phone and where? these transit areas are not equipped for such cases.
If somehow you managed to meet your friend, most likely she will be in a plane leaving the Schengen area totally, other than that I am not really sure.