The basic steps, that should happen:

1) Click the login button from the top right of this page.
2) Login with your SE Account (where your active sub is attached)
3) New members will be prompted with name creation and then selection of the active character from the game.

What exactly happens after she tries to login? Nothing? Error?

The only thing else to do different is to log into the Lodestone first and then try to enter the forums from there.

Just my guesses...

(and I'm assuming that her game account has an active subscription... since that's required for forum use, unless that's changed...)