She could fly to Canada and enter the US by land. To enter Canada, she can use her US birth certificate, meaning she won't even need to pay for an eTA (if she doesn't have the birth certificate on her, of course she can use her British passport with an eTA too)
The alternative would be applying for an ESTA on her British passport, however she'd have toshe should deny holding dual nationality, or the ESTA willmay be refused. I also can't guarantee that the system won't find out about her US citizenship.