I am so sorry the Doctor blew her off today. Most everyone here and in the POTS and EDS community has been blown off time and time again. It took us over a year to get a proper diagnosis for our Daugther once she reached the bedridden aspect of untreated POTS. A leading Ped Cardiologist totally blew us off. It is very difficult to go through the illness and then have the added disappointment and worry that no one will help. I am very sorry your family is experiencing it also.
What other symptoms does she have besides fainting? What city are you in? (someone may have a doctor suggestion). Do you know what her pulse rate and BP are while at rest, then after sitting and then again after standing up?