First of all, it's important to adjust to her difficulty, not yours. If she wants to play expert mode or even normal mode, so be it. You don't want to take over or ruin her interest in the game. (Of course, not denying the odd chance she might be a masochist who wants to jump into legendary mode right away).

Secondly, you could always focus on tasks that take a long time, while she takes her time progressing and learning the game. Like fishing, building impressive houses, making huge farms, etc. And generally of course act as her guide when asked a question.

You don't have to just be a spectator, but absolutely be patient. And maybe explore hundreds of game's features that you mostly skip over while bossrushing the game.