Two parents being sued over a high school birthday party held in their home nearly a decade ago told their side of the story in B.C. Supreme Court on Friday, saying they couldn't have known two teenaged guests would get into a deadly rollover crash on their way home.

Stephen and Lidia Pearson told the court they had strict rules in place the night of the party on Salt Spring Island, B.C., in 2012.

The survivor of the crash, now in his mid 20s, is suing the couple for negligence, claiming the opposite: he alleges the adults of the house should have better supervised the party and done more to ensure he and other teenagers made it home safely.

Stephen and Lidia Pearson, both dressed in dark suits, told the court Friday they were home during the party and thought it was just another run-of-the-mill, rural house party.

"The number one rule was no drinking and driving ... I trusted them," Lidia Pearson said, testifying at the end of a weeks-long civil trial.

The trial could set a precedent for any parent who has teenagers drinking in their home in B.C. There is no conclusive trial judgment in the province dictating the level of responsibility parents, as hosts, have over teenagers guests who drink underage; a B.C. Supreme Court judgment in the Salt Spring case could set that bar.

None of the allegations in the lawsuit has been proven.

Permission granted for birthday party

Calder McCormick, then 17, suffered a severe traumatic brain injury in the crash after the party on the northern end of the island. The impaired driver, Ryan Plambeck, was killed.

McCormick sued the Pearsons in 2014. The couple has denied all of the allegations.

On Friday, the Pearsons said they gave their daughter permission to have 30 to 40 people over to celebrate a friend's birthday. The couple said they had a zero-tolerance policy for drinking and driving, told teenagers to arrange a safe way home ahead of time and "delegated" their daughters to take away drivers' keys.

Both were adamant they did not provide the alcohol, except a two-litre bottle of cider for their twin daughters. They did, however, say they knew teenaged guests would bring their own drinks.

Lidia, 56, said she was OK with the underage drinking because she saw her daughter's friends as "good, responsible" kids who didn't get out of control. In his testimony, Stephen said they had allowed their children to have house parties before, in part, because they were ostensibly better supervised than the alternatives on a rural island: bush parties or beach parties.

Stephen and Lidia said they spent much of the party in their room, walking through the house to check on things about once an hour.

"It seemed pretty positive, pretty upbeat," Stephen said. "Just kids having fun."

After being pressed in cross-examination by McCormick's lawyer, Michael Wilhelmson, the couple conceded the party was unsupervised aside from their occasional walk-throughs.

The car Ryan Plambeck and Calder McCormick used to leave the party on Sept. 15, 2012. The uninsured 1992 Subaru Loyale was towed to a lot after the crash. (CHEK News)

McCormick's lawsuit claims he and Plambeck, the driver in the crash, came across an unlocked vehicle after they left the Pearsons. The Subaru belonged to a family living nearby. It was for sale and uninsured, sitting unlocked on the property with the keys left inside.

A coroner's report found Plambeck, 18, was impaired by alcohol and marijuana at the time of the crash. The report added he did not have a valid driver's licence.

Stephen, the host, told the court he didn't know the car was lying in wait the night of the party, so he couldn't have known the teenagers would use it. He said he did not see McCormick or Plambeck leave the party.

The site of the crash on Sept. 18, 2012. (CHEK News)

McCormick's father, Michael, sat in the front row and took notes during the couple's testimony. The families on each side of the lawsuit knew each other for years before the crash, with their twin children born the same year. (Calder McCormick has a twin brother, while the Pearsons have twin daughters.)

The Pearsons' lawyer, Jim Doyle, said the crash and subsequent legal fallout has been devastating for all involved.

"It has hung over their heads," Doyle told the court on Friday.

To successfully make the social host liability case, McCormick's lawyer has to prove the Pearsons owed a certain duty of care to anybody hurt by their underage guests — namely, McCormick.

Chief Justice Christopher Hinkson will have to rule on a set of questions: whether the parents of the house should have known Plambeck was about to drive, whether they should have been able to tell the 18-year-old was impaired and whether what ultimately happened to McCormick was "reasonably foreseeable."

The trial is expected to wrap next week.