His guilty plea is in connection to a separate incident involving Tauiliili on the same night, the Samoa Observer reported.
Ash is due to be sentenced on February 22.
Two other men, Herman Westerlund and Suapaina Savaiinaea, have both pleaded not guilty to manslaughter charges against them in relation to the death of Tauiliili, a father of one.
A hearing for the pair is scheduled to take place in Samoa on June 10, the newspaper said.
Tauiliili spent several years in New Zealand after being granted a scholarship to study here.
He was a student at Canterbury University for some time, before transferring to Auckland to be closer to his partner and their 3-year-old son.
Tauiliili graduated from the Auckland University of Technology in December 2017, with a Bachelor of Engineering Technology.
He had been working as a civil engineer for Samoa's Ministry of Works, Transport and Infrastructure at the time of his death.