Sophie, an honours graduate in economics about to leave the next day for her dream job at the Treasury in Wellington, was stabbed and cut 216 times by Weatherston.
"I am sickened, to be honest," Lesley Elliott said about Christie's death. "I know what they've been through and I know what they're going through. I'm appalled."
She said she and her family were still trying to get over Sophie's violent death.
"If [Christie's] family saw it happen, that's awful. I know what that's like. It turns me over a bit when I hear these stories, it's horrible. I can just imagine how they feel.
"In their own home as well ... like us. It's supposed to be your safe haven."
Mrs Elliott said she did not want to tell Christie's parents, Brian and Tracey, how they should feel or what to do. But she understood how they might be feeling.
"They're probably totally and absolutely devastated, bewildered, wondering why. On reflection now, the thing they can cling to the most is support. There is nothing more they can do; they'll be so devastated.
"For us, it was just a blur. We just got through. Everyone told us what to do, when to eat, when to sleep and we were just on remote control. It's pretty awful, it's horrible."
Christie was farewelled at an emotional ceremony on Saturday attended by hundreds of family and friends as well as police officers investigating her death.
Mourners were told at the service that Christie had been due to move to Australia this weekend.
Mr Marceau, an avionics technician who was in the air force for more than 20 years, has recently started working in Adelaide - and Christie and her mother were to have followed him..
The teenager was planning to continue her university studies there. Christie, who finished at ACG Parnell College last year, was in her first year of study at AUT University.
Akshay Chand moved to Auckland when he was a primary school boy.
