Ed Sheeran donated £10,000 to a hospice after an "on the off chance" request from a band he supported as a teen.
John Parker, part of folk duo Nizlopi, said he approached the star to ask for help in spreading the word about his fundraising marathon run.
The group had a hit single with JCB Song in 2005 and Sheeran credited them with teaching him "basically every aspect of my live set".
Mr Parker said he was "massively touched" by the donation.
"It proves that no matter how famous you get, you can still be a top person," he said.
In return, Mr Parker promised: "If he wants to do a marathon anytime, I have offered to help him."
Mr Parker said Sheeran started helping with guitars for the Leamington Spa-based band in 2005, when he was 14, and "just used to hang out with us."
He also opened some shows for them and Mr Parker said: "You could see he was very talented and knew exactly what he was doing."
Sheeran "just loved the music", he said, and was "clearly interested in the craft and how to perform."
The Galway Girl singer, based in Suffolk, spoke about his time with Nizlopi during a 2017 interview for Desert Island Discs and said: "Basically, everything from my live set, the way I control my voice, the way I control the audience, the way I play the guitar even, the way I used beat box… this comes from this band."
Mr Parker said the duo knew Sheeran was special, adding: "His level of growth was just unbelievable".
He said they had lost touch for a couple of years until he approached the singer "to ask for a tweet or something" to mention his virtual London marathon run in aid of Warwickshire-based Myton Hospices - who helped care for his late mother.
He said he had not expected much, because "everyone wants something from very famous people".
The donation was given under the name "Ted", but Mr Parker said he knew it was his old friend straight away and they have since chatted and exchanged banter about Mr Parker's running.
He said Sheeran "couldn't get his head around how far I run", but he suggested the star might want to take on marathon running himself, as a new challenge.