They can't look at doctor's notes. Go to City Hall, tell them what you need. I am not an expert in this, but I pay attention and try to learn from what others post, and from what I've read...probably getting her a wheelchair would be really helpful, and let them know she can't *stand* in lines so would need to take the wheelchair entrances.
Something that came to mind, if you go and there are short lines and she's up to standing, make sure you are aware that some lines have stairs where you'd least expect them. BTMRR and Splash have stairs well into the ride, and TSMM has them at the end as you're exiting. If stairs are out but she might want to stand in some lines, let them know that, too, so they can help.