Nikki Bumatay and her fiancé bumped into the songbirds while hiking at Fryman Canyon Park in Studio City, Calif.
"We were sitting in our car when this Maserati pulls up," Bumatay told PEOPLE. "I look back and see who gets out of the passenger door and it's Selena Gomez."
But that wasn't the only surprise the couple got on their adventurous outing.
"I look at the person who squeezes out of the driver side door [and] realize it's Katy Perry, my absolute biggest girl crush," she added of the singer, whose latest split from John Mayer was confirmed Tuesday.
Perry, who had an iced Starbucks drink in hand and wore "shiny pink lip gloss," was "very goofy and extremely gracious" when Bumatay approached the starlets for a photo, while Gomez (clad in shorts and a flannel) was a little more reserved.
"I wish I would have acted less like an 18-year-old fanatic and more like the mature 26-year-old I think I am," joked Butamay. "But it was a definite highlight of the day!"
– Melody Chiu
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