A random encounter with newly-reunited couple John Mayer and Katy Perry in a New York City guitar shop last week brought one lucky fan much more than just a great photo: Mayer actually bought the aspiring young musician a guitar on the sly.

On July 5, the superstars met a delighted 16-year-old Julie Fermin (far left, above) and her friends inside the store, cheerfully posed for a picture and talked music for a while before Mayer asked Fermin to point out her favorite instrument.

"I showed him, and he went over and checked the guitar's price," Fermin tells Us Weekly. "I said, 'Yup, but I can't afford it,' and he said, 'Oh, stop it!'"

Mayer and Perry soon left the store, but then a store employee stunned the girls with some unexpected news: Mayer had stealthily purchased the pricey instrument for Fermin.

"I was so shocked," Fermin says. "I started crying and screaming. I cannot describe all the emotions I felt at that moment. I just knew I was the luckiest and happiest person alive."

Here she is right after she realized what had happened:

 
And a shot of her with the guitar:

 
She also provided proof of who her generous benefactor was:

 
First he's painting houses for veterans, and now he's buying guitars for kids (and not even hanging around to soak up the glory)? Maybe Katy really has changed the notorious bad boy for the better.