The court heard that the 16-year-old boy saved Cashford’s number in his own phone as “pedo”, with Cashford believing he was in contact with Sienna, a fake name.
In messages, Cashford, 49, claimed to be 30 and asked Sienna if she liked champagne and said he wanted to kiss her, jurors were told.
A message supposedly sent by Sienna suggested they meet at her parents’ empty home and told him to bring alcohol, Blumgart said.
After Cashford met the girl, it is claimed that the boys followed the pair along the promenade, before the 16-year-old boy hit Cashford on the back of the head with a bottle.
Jurors were told that the girl shouted, called Cashford a paedophile and said she was 16, while filming the attack.
Blumgart said the 16-year-old boy was later seen by witnesses throwing large rocks at Cashford’s “already seemingly lifeless body and head”.
Jurors heard that the girl showed “unwavering enthusiasm” with her “shrieks” as she recorded the attack.
A post-mortem examination showed that Cashford had injuries to his face and head, bruises on his limbs and body, and fractured ribs that punctured his lung.
Prosecutors said the 16-year-old boy shared footage of the attack after he was arrested, with a caption referring to Cashford as “pedo”.
Jurors watched a video clip showing two boys and a man running along the seafront.
The trial continues.