Donna Abrey said the teen - whose has been named as Jamil Palmer - was a family friend and had been visiting her daughter at their home just before leaving for the park.
It is likely that she was one of the last of his friends to have seen him alive .
Police were called at around 3.30pm yesterday (Tuesday) to reports of a stabbing near Evans Grove, and an 18-year-old boy was rushed to a central London hospital.
The family later heard that Jamil was dead.
According to his Facebook page, Jamil was a former student at Richmond College, Kingston College and Teddington School. He lived in Belgrade Road, Hampton.
Mrs Abrey said they were 'devastated'.
"It's disgusting what happened. He was such a caring person to all his friends."
Meanwhile another resident said she regularly took her baby for walks along the path near where the murder is believed to have happened.
The shocked 35-year-old woman, who asked not to be named said, said she had not seen youngsters 'haging around' in the area before.
"It's shocking and disgusting that the crime had taken place in broad daylight," she said. "I'm sure someone must have seen something."
Two youths have been arrested in connection with the killing.
