The witness, who cannot be named for legal reasons, said he was an associate of Abedi and his brother Hashem Abedi - Salman being "a rough kind of guy" who would smoke cannabis and get into fights during his teenage years.
He lived a "gangster lifestyle" but became more religious in the year before the bombing, the Old Bailey heard.
But Salman's behaviour sparked concern from the teenager and his mother as his interpretation of Islam was deemed to be "too strict".
Salman detonated his suicide bomb on May 22 2017, killing 22 and injuring hundreds of others as thousands of men, women and children left the Ariana Grande pop concert.
Among those killed were Sorrell Leczkowski, 14, from Adel, Leeds; Courtney Boyle, 19, a Leeds Beckett University student from Gateshead; Kelly Brewster, 32, from Sheffield; Wendy Fawell, 50, from Otley; and Angelika and Marcin Klis, a couple from York.
Hashem was arrested in Libya, where he was allegedly tortured, the day after the atrocity before being sent back to the UK to face police questions amid prosecution claims he was complicit in sourcing and stockpiling components for the bomb.
He denies 22 counts of murder, one count of attempted murder encompassing the injured survivors, and conspiring with his brother to cause explosions.
In a witness statement read in court, the teenager, who is from the Greater Manchester area and knew the Abedis, described how he was warned about Salman due to his changing behaviour and religious views.
He said: "About a year ago (before the blast), Salman started becoming more religious. He started coming to our house more and having chats.
"My mum's view was his religious views were too strong and she told us not to listen to him.
"My mum would often confront Salman about his religious views and this sometimes would result in conflict between them.
"I'm aware Salman developed strong religious views. Me and my mum always told (a relative) that Salman's interpretation of Islam was too strict."
The witness said he had "more in common" with Salman, adding: "We have a mutual interest in football. Salman had a good left foot."
He continued: "In teenage years Salman was a rough kind of guy. He used to smoke cannabis. He would be violent, getting into fights, kind of like a bit of a gangster lifestyle.
"He grew up a bit, he became happier, more knowledgeable and was always smiling."
Jurors were told the witness was "unfit through ill health", and so could not give evidence in person.
It meant the defence counsel was unable to question the witness on his evidence.
The court previously heard the brothers allegedly duped friends and associates into helping to buy components of TATP, while switching vehicles and phones to ensure their actions went undetected.
They used an empty house to take delivery of the chemicals ordered on Amazon using others' bank details and fake emails, it was alleged.
The court heard how Salman previously asked the witness to make a money transfer to help a friend in Libya buy some doors.
The witness said in his statement: "Salman asked me a number of times to make this transfer. I could not see there was any harm in it and it was not my money and I was not losing anything so I agreed to do it."
The witness said Hashem drove him and Salman to the Halifax Bank in Chorlton, south Manchester, and Hashem waited in the car while the other two went into the bank.
The witness said Salman gave him £1,200 in cash which he paid into his bank account and he then asked the bank to transfer the cash to another account in China.
The witness added: "At that time I did not have any concerns this transfer was of any significance. I still don't know the purpose of this transfer. I just though Salman was helping out a Libyan friend."
The trial continues.