It's not often that a fan meets their music idol and receives a gift from them in the same day.
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John, 35, struck up conversation with the 16-year-old aspiring musician and him and Katy kindly posed with Julie and her friends.
After taking the picture, John asked the fan which was her favourite guitar.
"I showed him, and he went over and checked the guitar's price," explained Julie in an interview with US Weekly.
"I said, 'Yup, but I can't afford it,' and he said, 'Oh, stop it!' ".
"We started freaking out and getting super excited," she said of their reaction to meeting John and Katy.
After the couple left the Soho music store, an employee approached Julie.
"Well, you should get more excited, because John Mayer just bought you that guitar!" he said.
"I was so shocked," the teenager admitted. "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."
Julie couldn't believe the Your Body Is A Wonderland singer's generosity and quickly took to Twitter to show her gratitude.
"I never in my life thought I'd meet and get to talk to Katy Perry and John Mayer...", she tweeted.
"I literally thought it was impossible. FOREVER MIND BLOWN".
On Saturday, John began his latest tour in Milwaukee, dedicating his new song, A Face to Call Home, to the California Girls singer who was reportedly in the audience.
"To Katy, who is my face to call home," he said.
John described Katy's endless compassion for him during the early part of their relationship when she would talk for him during his recovery from throat surgery.
After a brief split for the second time in March this year, they seem to have been inseparable over the past few weeks, after several sightings in New York.