maybe you could go over her process of planning games and actually plan out the next game. Subtly give her pointers to make the game better.
This is what I was going to say, but I'd like to a amplify a point: She has to come to the Light Side (pun very much intended) on her own, but you can help. If she thinks the game is going great, you're just the asshole who doesn't like her style, or you just want your game back, or something. On the other hand, if she knows that people aren't enjoying the game, but doesn't know what to do, then you can provide suggestions.
I find that it works perfectly well to lay out a few options in archetypical form, both so that I can be surprised as a player, and so that she can put her stamp on the scenario. If you're just writing the adventure for her, then she won't grow as a GM or have much fun herself.
Be prepared for some continued rough spots, and also remember that you can't be too heavy handed. It works far better to present yourself as a resource ("Any frustrations with your game that I can help with?") rather than a scold ("You're sucking, and here's what you need to fix.") Just be prepared to do some work gently focusing the conversation so that she correctly identifies the problem -- "I designed an on-rails adventure that's too difficult for the PCs" rather than "The PCs aren't the right level."
As may be apparent, I've had a lot of these conversations. As long as you can stay focused on creating and amplifying positives, rather than eliminating negatives, they tend to go well. (Focusing on the negates tends to cast you as the scold, while amplifying the positives tends to end up crowding out the negatives. Eventually.)
Good luck!
