Thomasson had been in the care of the Henry Rongomau Bennett Centre, in Hamilton, but ran away from an escorted leave walking group on November 11, 2019.
He was found dead by police at 3.45am, near the Hamilton Central Police Station.
The court heard the woman had met Thomasson in the Henry Bennett Centre in October 2019 and once she was released, two months' later, they maintained contact.
During their time together they had discussed suicide and how they wanted to end their lives.
Thomasson was suffering from significant mental health issues and the pair maintained "extensive" text message contact which included him talking about his desire to end his life.
The woman told him that she was of "similar mind" to him, and could help them carry out their wishes.
The texts show Thomasson continued to seek reassurance from her about what they planned to do.
On November 11, 2019, the woman was at the Henry Bennett Centre picking up an associate when she saw Thomasson and encouraged him to abscond later that day when he was due to go out on a scheduled walk.
Thomasson did exactly that and went to meet her in her vehicle. The pair then drove to Cambridge, before heading to a grass reserve at 4.30pm.
At 3.15am, the woman phoned emergency services saying that Thomasson was unconscious. He was unable to revived.
'He was meant to be safe'
In her victim impact statement, Thomasson's mother said no matter what she said or did, her son was never coming back.
"This has happened and it really makes my stomach churn. Yes, Rhys was mentally unwell but you didn't make the right choice by helping him to do so.
"If anyone came to me I would try and help them in a positive way.
"Rhys was meant to be in a safe place and you helped him out of that safe place. He couldn't have done so without your guidance and help.
"Rhys lost his life and it has affected me and my family terribly."
Thomasson's father, Raymond, said the death of his son had left a "black hole in my life that can never be filled".
He said his son went to him, saying he needed help so that's what they did, starting off at Pathways before he was admitted to the Henry Bennett Centre.
He said his son needed to be at Henry Bennett to receive professional help and be safe.
"He should have been safe there. Your actions led to Rhys escaping from the Henry Bennett Centre and thereby enabling him to carry out his wish of committing suicide.
"Without your actions, Rhys may well be alive today."
'You knew what you were doing was wrong'
Justice Graham Lang told the woman she knew what she was doing was wrong.
At trial, he didn't accept her version of events that she had no idea what Thomasson was doing.
She was sentenced to two years intensive supervision and given permanent name suppression.
Crown prosecutor Rebecca Guthrie said the victim statements show the tragic impact the offending has had on the family.
Thomasson was "extremely vulnerable" at the time and wanted to end his life and through the woman's actions, he was successful.
The woman had texted Thomasson throughout the several days before his death, encouraging him and saying she had taken steps to help him "meet his goal".
However, she also acknowledged that she had her own mental health issues – having been diagnosed with borderline personality disorder which was partly the reason why she had lacked remorse for what's happened.
She urged the judge to take into account rehabilitative and therapeutic factors but also public safety.
The Crown and Thomasson's family both opposed permanent name suppression – the family because they wanted the public to know who was responsible so that it did not happen again.
The woman's counsel wanted suppression to continue to help with her rehabilitation and reintegration back into the community, and said it could have disastrous consequences if her name was publicised.
'We hope it brings comfort'
Police acknowledged the sentencing of the 29-year-old woman today.
Detective Simon Moore said Thomasson was unwell and "in the process of receiving treatment when his life was tragically cut short".
"Our thoughts today are with Rhys' family and friends.
"While the outcome today will not bring Rhys back, we hope that it provides some comfort to see the accused held to account for her role in his death.
"This has been a challenging case to investigate and prosecute and I want to thank the team for their outstanding work.
"We would encourage people to reach out to support services if they are struggling. It's okay to not be okay, but please reach out for help."
WHERE TO GET HELP:
If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.
OR IF YOU NEED TO TALK TO SOMEONE ELSE:
• LIFELINE: 0800 543 354 or 09 5222 999 within Auckland (available 24/7)
• SUICIDE CRISIS HELPLINE: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• YOUTHLINE: 0800 376 633 ,free text 234 or email talk@youthline.co.nz or online chat.
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757
• SAMARITANS – 0800 726 666.