No STD can cause genital soreness that continues without much change for several months, and cancer doesn't do that either. But that's all I can say; I have no idea about the possible diagnosis. She needs to see a health care provider and stop looking on the internet. There is no way she will ever get a clear answer by searching on line.
Good luck-- HHH, MD
While you wait for Dr. Handsfield's reply, why on earth hasn't she seen her doctor? It's ridiculous to go looking around on the internet, worrying about what it might or might not be. Just go get it checked already. If money's a problem/you don't have insurance, Planned Parenthood will work on a sliding scale, she can go to your health department, or she may qualify for services at your free clinic.