He said the two women had met when they worked together in China and kept in touch, texting each other once or twice a week.
"[The friend's] English is not the best, but she said, 'I don't know what to do, I don't even know the number for the police'," he added.
Mr Newman added that the note gave clues to the backpacker's location - as she revealed she had been taken two hours from Murray Bridge and also described going over two ferries.
A manhunt was launched after she also managed to get a desperate message to her family in Europe who alerted police.
Police swooped on the alleged kidnapper's farmhouse in Meningie at around 12.30pm on Saturday.
The 52-year-old was taken to the Murray Bridge police station and interviewed at length.
He has been charged with three counts of rape and one count of unlawful detention.
The man has been refused police bail and is expected to appear in the Adelaide Magistrates Court on Monday.
The backpacker, who cannot be named for legal reasons, had left her accommodation in Adelaide's CBD and boarded a bus to Murray Bridge on Thursday morning.
She was picked up by the man when she arrived, and police allege that he drove her in his red Toyota Hilux ute 77km south to a property in Meningie, where she was sexually assaulted.
She was discovered at a motel in Murray Bridge on Friday afternoon after a tip-off from a member of the public.
The well-travelled woman arrived in Adelaide early in February after stints in Tasmania, Sydney and China recently after visiting other parts of the world over the past few years.
She posted an ad on Gumtree earlier in the week along with a picture of herself feeding a kangaroo, saying she was looking for a farm or fruit-picking job.
"I'm looking for these jobs in Melbourne starting 30 of January. With accommodation stay or hostel nearby. Cuz I don't have a car," it said.
"I love nature and animals, I eat fruit every day;) it would be nice to do a job like this. If you have something please let me know. Thx."
Meanwhile, Premier Jay Weatherill fears the alleged sex attack could put overseas tourists off from visiting South Australia.
"There's a significant reputational issue here," Mr Weatherill said on Sunday.
"But people should be rest assured that perpetrators of crime will be detected and they will be held to account and appropriately dealt with."
He also called the matter an "awful incident" and said he hopes it will be investigated quickly.
"We don't want to see any individual in our state the victim of a crime," he said.
"These are awful incidents and we want to eliminate them entirely if we can."
