The ESTA does not apply when entering in a boat, so your friend will need a B1 Visa. There is a workaround when entering the USA in the USVI; one can take the ferry
from the BVI to St. Thomas or St. John using the visa waiver, then keep the entry form and return to the BVI and sail the boat into St. John and clear in there. But if coming from across the Atlantic the visa is mandatory, unfortunately. The B1 application should be done in the country of residence and there is a written application to complete and subsequently an interview at the designated U.S. embassy or consulate. Plus a hefty fee. And waiting time while they have your passport to put the visa into.