Quote Originally Posted by PetiteMalFleur View Post
Just a bit of advice, DON'T USE STEAM!

Steam is considered a different platform so she cannot buy the Steam version of Heavensward without first owning a Steam version of ARR. However, once she ties her account to Steam she will forever have to do everything through them. SE will not unlink your account from Steam and the only way to play the game without the Steam affiliation is to start a new account.

Just be glad she is not on Steam because they are in a mess trying to get the Heavensward purchases working. There are a lot of very upset players who have no way to get access to the expansion until it gets worked out with Valve. She would also not be able to get physical collectors editions unless Steam offers it, which they are not doing for Heavensward: another sore subject for stream users.
I came here wondering the exact same thing. Thanks for saving me the trouble!