Exactly. You learn how to write by writing. She has to do the work to get better (and how great is it that she loves the work!). A book with writing prompts and tips would be better, because it could be the source of inspiration and advice without being adaptive and doing the work for her. Good old-fashioned non-adaptive resources (books!) will empower her to follow her own creative instincts. Also, writing is intensely human – technology doesn’t make one a better writer.
OP, you don’t have to have mastered a skill or craft to encourage your kid as she pursues it (that’s part of the fun, really – I’ve learned so much from my kids, who’ve developed interests completely outside of my areas of expertise). You’ve already gotten lots of great ideas, but thing I want to reiterate is that great writers develop their talents by becoming great readers. Stock her shelves and get her a library card so she can derive inspiration from what she reads. That goes for you, too – if you want to learn a thing or two about writing, start reading more on your own (not suggesting you don’t).
I don’t know what resources are available to her in middle school, but when she’s in high school, encourage her to join a creative writing club, the school newspaper, and/or a school literary magazine if there is one. If any of these resources are also available in middle school, then now is a great time to start!