Neil McFerran and Aaron Davidson were both 18 when they were overcome by the deadly gas in a holiday apartment in Castlerock, Co Londonderry on 3 August 2010.
The pair - who along with another friend Matthew Gaw - had gone to the seaside town for a break ahead of getting their A Level results - were found by their parents, who rushed from their homes in Newtownabbey to Castlerock when they were unable to contact them on the phone.
Katrina Davidson said initially she hadn't been worried as her son usually had his phone on silent.
But when Neil's mum Catherine said she hadn't heard from him and that she had expected them home, that was when "panic set in".
They went to the holiday apartment in Castlerock, and saw things strewn about as if the boys were still inside.
While waiting for the police, they managed to break in through the bathroom window.
"When first we got through, it was so surreal, we went into the living room, and I saw Aaron lying there, I thought: 'Something's not right'," she told UTV.
"I just didn't believe that he had died, I actually thought it was meningitis because I remember seeing his t-shirt - under it he was pure purple.
"I remember Johnny and Catherine finding Neil. Poor Matthew was just sitting there in awe, he was conscious but barely.
"I still thought that this isn't happening to me because people like me don't lose children."
Katrina said the friends had made plans to go to New York that September before going onto university, where Aaron wanted to study engineering.
They had high hopes. Aaron couldn't wait to get his A Level results. That day came and he wasn't here.
She said that results day was difficult, as her son had worked so hard to do well in his exams, and he did - achieving an A*, B, C, and D.
"Looking back on it now, I don't know I coped, I remember just thinking: 'I will take every day as it comes, just get today over'."
On Tuesday, gas fitter George Brown, 52, from Ballygawley Road in Aghadowey appeared in the dock of Coleraine Crown Court, sitting in Belfast, where he pleaded guilty to unlawfully killing Neil McFerran and Aaron Davidson.
He admitted 21 charges which also included a string of offences involving failures in health and safety legislation.
Both Aaron and Neil's families were in court.
Katrina said she was relieved there would be no trial and that Brown had admitted responsibility.
"We were right, he did do it, he is responsible for taking Neil and Aaron and justice has finally happened," she said.
"As far as I'm concerned, no sentence is appropriate - if anybody else lost their son or daughter through somebody's negligence, obviously you want them locked up.
"Everybody would say 'forever and throw away the key', but I know that's not going to happen, I think justice was far more important."
Katrina remembers her final moments with her son.
"It was just a weekend break in an apartment he had stayed in before, you just can't imagine (him) leaving the door and not coming back," she said.
"I said: 'Enjoy yourself - love you, son' and he just kissed me as he always did and said: 'Love you too, mum' and that was the last time I saw him.
"My life will never be the same without him. That man took away Aaron's life and his future - he took away ours as well."
Since their sons' tragic deaths, Catherine and Katrina and their families have worked tirelessly to ensure every home in Northern Ireland has a carbon monoxide alarm.
Katrina added: "We have to show other people they need alarms, they need to get everything serviced."