Jake Sexton, 27, joined Williams and three others to confront Ryan Naumenko in July 2019 after the television project fell through.
Mr Naumenko promised Williams, the former wife of murdered underworld killer Carl Williams, that he would make her lots of money but she claimed he failed to pay up.
Williams enlisted Sexton and the others to blackmail the producer, luring him to a Collingwood studio on the evening of July 9 that year.
Sexton and one of the other men repeatedly punched the victim to the head while Williams yelled out, "kill the c*** - he has no money".
Over three hours Mr Naumenko was continuously assaulted and threatened.
He was forced to call his father and sister asking them to each transfer $1000 to the offenders, which they did.
Mr Naumenko was also made to sign a document transferring the ownership of his mother's car to the group.
One of the offenders then drove Mr Naumenko home and ordered him not to tell the police. Williams later sent the victim a message thanking him for his assistance.
Mr Naumenko sustained numerous bruises and cuts to his face and scalp during the attack.
His jaw is still out of alignment to this day and he struggles to go out in public, the victim said in a statement.
Sexton on Monday pleaded guilty in the County Court in Melbourne to one charge of intentionally causing injury.
The 27-year-old admits assaulting the victim but he did not make any of the unwarranted demands or threats, his lawyer James McQuillan said.
"He was called in at the last moment to make up the numbers without knowing what it was all about," Mr McQuillan told the court.
"He foolishly and naively agreed to partake."
Although Sexton had relevant prior convictions, Mr McQuillan said he was still a young man with stable employment.
He did not receive any financial reward from participating in the attack and instead was only following orders, the lawyer said.
A community corrections order with unpaid work conditions would be an appropriate punishment, Mr McQuillan submitted.
Sexton was released on bail ahead of his sentence hearing on Friday.
Williams earlier this year pleaded guilty to charges of blackmail and recklessly causing injury. She will also be sentenced later this week.
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