French has already served about four years behind bars as part of his life sentence for murder.
Supporters of Sears, also known as Luke Riddell, 28 at the time of his death, were in court to see the guilty plea.
The Crown and defence have not yet finalised a summary of facts, but defence counsel Andrew Bailey said it would be completed within a fortnight and presented to the court for the sentencing.
He and Crown prosecutor Mitch McClenaghan said the details in the summary would be taken from the murder trial which was held in March 2020.
At that trial, Alistair Cochrane, 28, and French were convicted of murder and Cochrane continues to serve his life sentence.
Sears was shot in the chest on Grange Rd, at Charing Cross, west of Christchurch, on October 13, 2018, as a result of a conflict over an alleged drug debt.
French had been running a large-scale cannabis growing operation in the area, and Sears had bought cannabis from him. French claimed Sears owed him $20,000 but Sears thought French owed money to the King Cobras gang.
French and Sears confronted each other on the roadside and Cochrane then pulled out a black shotgun. Sears advanced towards Cochrane asking him what he was going to do with the gun.
The trial was told Cochrane said he was leaving, but Sears then tried to take the keys from the vehicle and Cochrane then pulled the trigger, shooting him in the chest and shoulder.
Cochrane and French drove off, leaving Sears dying by the roadside, with his fiancee desperately trying to get help.
Crown prosecutor Mitchell McClenaghan argued at the trial although French was not the one who pulled the trigger, he was guilty of murder under the party liability rules.