Prosecutors said Cashford was chased and hit with rocks and a bottle before he was found lying face down in the mud.
A post-mortem examination showed injuries to his face and head, bruises on his limbs and body, alongside a number of fractured ribs that had punctured his lung.
The 16-year-old boy had earlier told the court how he and the others had discussed giving Cashford “a slap” before the attack.
He claimed he attacked Cashford because he thought police “wouldn’t have done anything” if they had reported the man for trying to meet up with the girl.
“This was a carefully pre-planned deliberate and violent attack on someone who was not expecting it and who could not defend himself,” said Natalie Smith, senior crown prosecutor from the Crown Prosecution Service.
“Despite his best attempts to flee, he was relentlessly pursued and attacked, even when witnesses reported he was lying on the ground.”
The three teenagers – who cannot be named for legal reasons – were acquitted of murder on Monday.
They are expected to be sentenced for manslaughter in April.
“These verdicts will change their lives forever,” said the judge Mrs Justice Cheema-Grubb.
