I actually have the same thing, or something similar that causes the same reaction. Is what she has called neurocardiogenic or vasovagal syncope? They're the same thing, just different names and that's what I have.
I wrote to Disney over this, because it's exactly the same where standing still too long will make me faint. They replied with the general suggestion of a wheelchair/
ECV.
She'll probably find that heat will make her more sensitive. Really she's going to have to figure out how sensitive she is with it, how easily that reaction comes on. Moving around a bit, shifting her weight from foot to foot, squatting down for a bit, things like that, will help. If she finds that very short times of standing will bring it on, she's going to be best with a wheelchair/ECV. Preshow areas were hard for me unless I was by a wall where I could lean against it, but I didn't feel the need for a wheelchair. She may also ask her doctor if a small dose of beta blockers would be appropriate. That's what they put me on, and the purpose was so that my blood pressure never got high enough to trigger this reaction (again I'm assuming she has the same thing or something very similar to what I have). A GAC is not intended to reduce wait times, nor will it provide her with any sort of seating in line if she were to feel faint. It is against Disney rules to bring in any time of folding seat, but I believe a product called the Rollator is allowed (Sue and Kate feel free to correct me here). The Rollator is essentially a walker with a small bench type seat attached between the bars.
Since it's still early in the year, it shouldn't be extremely hot out so that should help. She'll obviously want to take lots of breaks and keep well hydrated but may find herself more fatigued than normal. There's ample places around that she can escape from the heat in, so if she finds the heat is making things worse she'll can take refuge inside.
Another tip that I find to work is to let her be first in the party in lines. That way she can let the line move up slightly and create a very small gap that she can move around in. I'm obviously not talking about a 10 foot gap in the line, but a good foot in a half in front of her should be enough where she's not standing stock still. Then the people who are with her can move up at a pace that would keep that small gap in place.