He was taken aback when she returned, intent on building a children's home in Tamil Naidu.
"She'd met this guy on a bus who told her about an orphanage he was starting up," said Mr Watson, now living in Solway.
"It would only have had about two or three kids at the time.
"She came back and started raising money, got all these people behind her and put a trust together.
"Mum had never shown any interest in working with kids that I saw - it was a chance meeting which changed her life."
Eventually, she sold her house in Aro Valley, returned to India and purchased the land for the Karunai-Illam Children's Home where she became "Aunty" to the hundreds of children who passed through its doors.
She wrote to the young Mr Watson about her adventures which he confesses he didn't find exciting. "I was a teenager, and hanging out with a few ruffians, so I didn't really want to hear about all the cute little children she was meeting," he said.
"But when I saw her work on screen, I was very proud."
Aunty and the Star People follows Ms Watson's work at the Illam, her relationships with the children, past and present, her helping build a new girls' home and school on the land, and her contributions to the local community - such as gifting saris to women in a rest home on her 80th birthday.
The film also touches on her semi-autobiographical novel Stand in the Rain and her 10-year relationship with Barry Crump.
Mr Watson said the film was "really neat".
"It was visually spectacular; an understated, very human piece.
"If I wasn't connected to it, I would still go and see it."
Mr Watson said he was close to his mother growing up and clearly remembers her eccentricities. "She was always spouting poetry, and was very much into obscure religions.
"She had all these tracts in the house, with sayings like, 'Trust in God, but tie up your camel'."
He believes his mother is truly selfless. "She never expected payback for everything she's done.
"People call her a saint, which she rejects - but then again, any saint would."
Aunty and the Star People will screen at Regent3 on Thursday, October 23, and is now screening at Circus Theatre.