Fermin, 16, was browsing the selection at Rudy's Music in New York City when Mayer, who had performed there, and his girlfriend Katy Perry entered the shop, reported People magazine.
"I started crying and screaming. I was the luckiest and happiest person alive," said Julie Fermin, an aspiring musician.
"(My friends and I) started freaking out and getting super excited. They were just so comfortable and sweet and as nice as any couple," she said.
Mayer, 35, also casually asked Fermin how long she had been playing and to show him which guitar she liked best. After that the couple left the store and Fermin was told Mayer had purchased the guitar for her on the way out.