The pair grew up together, first playing as under-8s and all the way to State of Origin.
But when his playing career ended and he moved out of the spotlight and behind the scenes, Greenhill believes that's when Green's struggles began.
"It's been there a long time," Greenhill said. "He first spoke about it probably 18 years ago.
"The NRL, it's like a machine that can just spit you out. They [officials] are ruthless and they'll throw people under the bus because it's a result-driven business."
Greenhill said Green was not tormented by demons constantly and "was generally happy".
He added he loved his wife and kids, his family and was financially stable.
Speaking to The Courier Mail's Peter Badel, Green's 82-year-old mother Patricia and 93-year-old father Ned revealed their utter devastation at the loss of their child.
"We never expected to bury our baby boy," Patricia said.
"His father Ned is just broken-hearted. We will survive somehow, but I will never understand it.
"I am just stunned, devastated, broken hearted. I hope the feeling goes one day. We have cried ourselves out. There are no tears left. We are a tough old generation, I guess, and somehow we have to move on.
"But our lives will never be the same."
Patricia also revealed she couldn't believe the news when she was told by her other children.
She said Paul was a "man of faith" and she "couldn't believe he would do it [take his life]".
She also added she saw no signs of depression the last time she saw him — his son's 10th birthday, which was just hours before his death.
Patricia said they had "sat down and laughed and talked and then we went home."
"I had just left Paul a few hours before it happened. I kissed him and said goodbye, 'I love you Paul, catch ya later'. Then off we went home," she said.
The grieving mother said she "never thought" her son was struggling to the extent he would take his life, and told the Courier Mail there was no difference in how Green had acted.
"We couldn't fathom why he would do it. We still can't," she said.
"It is against his beliefs. It is so hard to accept."
Green had a strong coaching career, leading the Cowboys to their maiden premiership over Bennett's Broncos in 2015, while also leading the side back to the grand final in 2017 despite losing Johnathan Thurston through injury.
But having left the Cowboys in 2020 after a run of poor results, before a season as coach of Queensland, which ended in a 2-1 loss, Green was looking for his next job.
Green had also been offered two assistant coach jobs for next year, including alongside Wayne Bennett at the Dolphins.
Despite this, the Sydney Morning Herald reported on Sunday Green had expressed interest in coaching the Gold Coast Titans.
On the Friday before his death, he even asked Mat Rogers about the Titans during a round of golf.
The report also said those who knew about his battles that he masked "how tough and frequent his dark days were".
"Green didn't want his mental health struggle to be widely known, in case it had an impact on his ability to get and hold another head coaching job," the report said.
Thurston, the captain of Green's 2015 premiership title, delivered an impassioned plea on Monday, calling for people to speak out and get help if they're struggling.
"Mental health has really come to the forefront the last 10-15 years," Thurston said.
"Before that it was an era of 'she'll be right, mate'. Well it won't be right! It's not going to be right.
"We need to be there for our mates and get them to talk about their feelings and what they're going through and [get them to] seek professional help if they need it.
"The days are gone of 'she'll be right', of suicide taking far too many lives in this country.
"We need to be able to talk about it."
He also called Green "the rock for everyone else".
"He was saying, 'Give me a call if you need to chat. You've got so much to live for. You've got health, family, kids'," Thurston added.
"So it just doesn't make sense."
A public memorial will be held for Green on Tuesday, August 30, and is expected to be well attended.
Speaking about the memorial, Green's brother Rick said the event would take place at Kougari Oval and invited the public to come and celebrate the league great's life.
WHERE TO GET HELP
• Lifeline: 0800 543 354 (available
24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youth services: (06) 3555 906
• Youthline: 0800 376 633
• What's Up: 0800 942 8787 (11am to
11pm)
• Depression helpline: 0800 111 757
(available 24/7)
• Rainbow Youth: (09) 376 4155
• Helpline: 1737
If it is an emergency and you feel like you or someone else is at risk, call 111
