Imogen Potter and her son Rupert – an Earlston High School S5 pupil – had travelled to enjoy the ceremony as a pre-arranged post exams treat.
It was all going so well, with visitors enjoying the display by the armed services bands.
It was a big treat for Borders born Rupert, who had army cadet experience when the family briefly lived in Northumberland before returning to the Borders, which may have prepared him for what happened next.
Rupert, 17, who is autistic, is normally “quite a shy lad”, but he handled what was about to happen with a great deal of calm and focus.
Imogen takes up the story.
She said: “At 11am the crowd was told to, as is Army tradition, ‘All Stand’ for the National Anthem.
"There was a loud bang, a scream and two of the three women sitting directly in front of us simply disappeared as the stand’s floor collapsed.
"Rupert used his previous cadets experience to act swiftly.
"He shouted: ‘move!’ to me, and so quickly reached a third woman who was waist-deep over a 30ft drop through the scaffolding to the ground below, where the other two women had already fallen.
"Another man had grabbed her first, but as he held on he struggled to balance in the task. Rupert was there to help steady and the woman was pulled to safety.
"Were it not for his quick actions, the result may have been much worse. He didn’t have time to get anxious.
"He was amazingly calm and very focused. I’m so proud of him, but he’s very modest about it all.
"He’s had a difficult few weeks, what with his Highers and the death of his grandmother in Kent affecting him.
"We are travelling down again for her cremation next week and will be passing through London on June 2, but if we see the procession again it will be from the pavement this time.”
A total of five people were injured in the incident, with three needing hospital treatment.
The Army said a “partial structural failure" was to blame for the collapsing floor.
Imogen said: “I’d asked Rupert to concentrate his energies on his exams this year, but the incident has clearly shown how appropriate it would be for him to re-join his local Army Cadet detachment, exam years or not!”
Rupert told us: “It’s strange to hear people labelling me as ‘brave’ when I just followed my training and did what any other cadet would do in an emergency situation. I really hope the people who were injured are okay now.”