In a new book, "Secrets of a Marine's Wife: A True Story of Marriage, Obsession, and Murder", Shanna Hogan explains how that new "friendship" would lead to Erin's death.
Their next-door neighbours were Chris and Nichole Lee and their 6-year-old daughter, Liberty.
The Corwins, still trying to come to terms with life as young newlyweds, could often be heard fighting about money, accusing each other of overspending.
Not long after they'd moved, Erin announced a pregnancy on Facebook but ended up miscarrying weeks later.
This had a strong impact on their marriage. She withdrew from social life and became increasingly distant from Jon, who didn't know how he could help his wife.
Then she caught Chris Lees' attention.
After the couples hung out one night in one of the apartments, Erin and Chris began an affair.
Nichole was quick to notice her husband had become distance. She checked his phone and found the messages he'd exchanged with Erin.
She confronted her husband and then told Jon about it too, in front of everyone.
"As Erin turned to leave," Hogan writes, "Nichole stormed after her, eyes flashing with anger. She shoved her finger in Erin's face."
"If you ever have anything else to do with my husband, I'll kill you myself!"
The affair was briefly interrupted but, a few weeks later, Erin and Chris were seeing each other again.
Things escalated quickly.
On June 22, Erin found out she was pregnant again. There was no social media post about it this time.
A week later, Chris planned to go on a trip to hunt coyotes. He packed his .22 rifle.
Erin happened to be visiting the same national park. She left home around 7.30am to head out scouting places to then take her mum to on an upcoming visit.
She was never seen alive again.
An investigation into her disappearance quickly became a murder one. Chris and Jon were initially both suspects but Jon was ruled out early on. Chris made a few mistakes, including lying to police about the affair.
Police found he'd done Google searches on how to dispose of a body.
However, it took police several weeks to find Erin's body.
In August, a caver noticed a strong gasoline smell coming from one of the mines. It was there that they would eventually find her body.
Police found she was choked to death with a homemade garrotte, then dropped down a mine shaft.
A propane tank was found next to her and authorities believe this was part of an effort to burn her body.
Chris Lee was arrested and charged with first-degree murder.
Police then found Chris did meet Erin at the Joshua Tree National Park the day she went missing. In fact, he had previously told her he had a surprise for her there and lured her to the park where he'd eventually kill her.
A trial with a twist
Chris Lee's trial began in October 2016, with defence presenting just one witness: himself.
He took the stand and confessed to killing Erin - but also made some other shocking statements.
While on stand at his own trial, he said that while speaking to Erin about her desire to become Liberty's stepmum, he remembered one particular episode.
"One night after his wife had given Liberty a bath," he told court, "Nichole found her daughter's crotch to be red and irritated. Nichole … accused Erin of somehow causing the 'suspicious' rash."
He said that made him realise Erin was too emotionally attached to Liberty and that "turned a gear" in his head.
"Did you touch Liberty?" he said he asked Erin in the park. "Did you molest my daughter?"
According to him, she said "yes but" and that was the last thing she ever said.
Court considered this was his attempt to be considered for involuntary manslaughter rather than murder.
However, jurors did not believe him. It took them 15 minutes to reach a guilty verdict.
He was sentenced to life in prison without possibility of parole.
