I like weeblewobble's post a lot. And the wording--you could even have "I don't know. It's up to you. I'm just the proofreader" in a text file somewhere, and copy & paste it into the chat box whenever she asks you a question about plotting, dialogue, etc.. Just keep doing it--it might take a while, but she will either get the point and stop asking, or YOU will decide you don't want to deal with her at all and sever the relationship.
Actually, I would suggest not chatting with her online anymore. I mean, if she's actively writing, she doesn't need to be asking you questions about anything in real time. She can write a couple of pages and email it to you, then you mark it up and email it back. If she doesn't understand a change she can email you again. You can answer her emails at your leisure, and if she emails you questions about the direction of the plot, you can still respond with the stock phrase from above. If you still want to help her as a proofreader, it might help your level of frustration to take away the immediacy of chatting.
I'm sure you're being clear with her. But it's not what she wants to hear, so she's trying to bulldoze through it, hoping you'll change your mind and give in.