Kerry Roberts' daughter Leah Heyes, 15, collapsed in a car park after taking MDMA with friends in May 2019.
The teenager of Northallerton was rushed to hospital but tragically died.
Connor Kirkwood, then 17, was arrested for dealing her the class A substance. He was jailed for 21 months last year but after serving six months of his sentence was released.
Ms Roberts said she realised both her and his mother, Tammy Kirkwood, had "lost something" and the pair are now friends who are raising awareness of the dangers of drugs.
Speaking to the BBC, Ms Roberts said: "People will look at us and think it's an unlikely friendship. They'll see us as two separate situations but we are both grieving - they are both our children."
The mothers were introduced to each other through restorative justice - where victims of crime can meet those who committed the offence.
At 15, Conor had become involved in county lines gangs, which target vulnerable teenagers and use them to supply drugs.
Speaking to the BBC, Tammy said: "I thought where have I gone wrong? How did this happen? What did I do? I'm constantly carrying the guilt of my child being involved in someone else's child losing their life.''
Leah's mum was initially against the meeting, but says hearing the other side of the story helped ease her emotions of anger and hatred for the situation.
Both women have now teamed up to work on on a campaign called 'Do You Know MDMA?' to get the message out to young people that drugs kill.