She could always implore the surgeon to stuff as much as he can in there on surgery day. But, if she's getting silicone, then either the surgeon would have to test with saline style implants (ie, filling with air is often used just as a quick sanity check, if they test at all), unless he's exceptionally good at just eyeballing it. My guess at this point is that the surgeon has 500cc stuck in his head, a bit cookie-cutter, but 500cc is better than 250cc. Silver lining, and all that.
If she can't get more than 500cc the first time out, then she can consider the first surgery as being the one to get past all the rough, uncomfy stuff. Once the pockets have been established and the body is accustomed to having those foreign objects there, upgrading should be less bothersome. It would be an unfortunate delay (one which I can totally appreciate), but so long as the surgeon would be open to helping her go bigger later on, then great.
Actually, there's a thought: has the conversation regarding a future upgrade taken place with the surgeon yet? The surgeon might reply with "well of course I would help you go bigger if you're not satisfied, etc, but give these some time blablabla", hoping that it was simply a case of "she doesn't really know what she wants, I know my patients". I think some surgeons are genuinely surprised when what they think is plenty turns out to be not quite what the patient wants. The surgeon may even try to talk her out of going bigger, so brace for that. If that happens, then it's time to start looking elsewhere. No point trying to negotiate when the other side isn't open to negotiations.