London Bridge victim Jack Merritt was stabbed to death when he confronted killer Usman Khan in the toilets, it has emerged.
The horror unfolded inside Fishmonger's Hall where the pair attended a Learning Together programme to help rehabilitate former prisoners.
Lisa Trayers told Sun Online how her former partner John Crilly, who helped tackle the killer with a fire extinguisher, recounted the drama to their son Warren.
She said: "They were at the meeting and they noticed the terrorist had gone missing.
"They couldn't continue without him so they went to find him. Jack has gone into the toilet - that's when he's claimed his first victim and stabbed back.
"He must have been strapping himself up. Khan was very quiet and didn't say much and when he's come out of the toilet that's when he's said he was going to blow the building up."
Second victim Saskia Jones, 23, was stabbed when the group started throwing chairs at Khan.
Jack, 25, and fellow Cambridge graduate Saskia were both course co-ordinators.
Lisa, 48, said Jack had driven Crilly and Khan to the event last Friday.
Crilly had been convicted of murder and later changed to manslaughter on appeal and was released from jail last year.
Lisa, who split from Crilly more than 20 years ago, said: "He just want to get on with his life, better his life and help others.
"He's trying to help other prisoners. It's in his nature to help.
"Jack and Saskia used to go into the prisons and that's how Crilly met them."
Jack's friends are now fundraising for 'one hell of a celebration' in his memory.
The GoFundMe appeal has already raised more than £13,000 - with pals saying additional money will go to causes close to his heart.
The appeal says: "We are all devastated by Jack's death but determined to celebrate him.
"Jack loved music, art, eating good food with his family, and having more than one pint with his mates.
"And on that point we, his friends and family, are going to throw one hell of a celebration for him.
"Because my god, did that boy know how to party."
Following the attack, Mr Johnson and Home Secretary Priti Patel said there would be a crackdown on early release of convicted terrorists.