Mr Heras was riding pillion on a rented 400cc Yamaha motorbike being driven by his friend Ludovi Didier Donny, 37, who died in the smash on the seafront road in Karon.
The two men had arrived just hours before to begin their holiday in Phuket.
An appeal went out on Friday for rare A-negative blood to allow surgeons to perform operations to save Mr Heras, who had arrived at the hospital in a comatose state with severe bruising to his brain and lungs, a broken jaw and multiple fractures to his left leg.
Despite the success of this appeal, however, the surgeons were unable to save his life.
An ICU nurse told The Phuket News that Mr Heras was declared brain-dead in the early hours of today. His body is being kept at the hospital while arrangements are made for its repatriation.