Lenin Gutierrez, 24, was working at a Starbucks in Clairemont when Amber Lynn Gilles entered the cafe without wearing a face mask.
Mr Gutierrez, a college student, refused to serve Ms Gilles, pointing out the Starbucks' face mask policy which asks all customers to wear a face covering to prevent the spread of coronavirus.
Ms Gilles, who says she has a medical condition which means she cannot wear a mask, left the coffee shop.
But she later shared a picture of the student on Facebook with the caption: "Meet lenen from Starbucks who refused to serve me cause I’m not wearing a mask.
"Next time I will wait for cops and bring a medical exemption.”
But, instead of people empathising with Ms Gilles, social media users rallied behind the barista and slammed the customer's behaviour for refusing to wear a mask.
Matt Cowan, an Irvine marketing and brand strategist, then started a GoFundMe campaign to raise tips for Mr Gutierrez.
His aim was to raise $1,000 for the barista, but donations flooded in and a total of $105,450 (£84,000) was raised.
Mr Gutierrez told The San Diego Union-Tribune: "I never seen anything like this except in the movies. It’s more money than anyone in my family has ever had. I feel like I’ve been given this incredible opportunity that I never saw coming and I don’t want to waste it.”
But Ms Gilles is now claiming that the student's decision to serve her was discrimination because of her medical condition.
She shared two documents with ABC 10News that she says is evidence of a medical condition.
One of the documents is a pelvic exam from 2015 of ovarian cysts and the second is a handwritten note from a San Diego chiropractor who asked to no be named.
It reads: “Amber has underlying breath conditions that prevent her from wearing a mask or any type of facial covering whatsoever. Please contact me if have any questions.”
Ms Gilles told KGTV: "It was discrimination and everybody is okay with it and enabling and rewarding that behaviour."
Speaking about her symptoms from her medical conditions, she said: "One of them I get shortness of breath, dizziness and it messes with the heartbeat. And I do have asthma as well, and I do get mask-acne.
"So there’s several things going on and not only that but it doesn’t even work.”
Ms Gilles said she consulted lawyers about a case for her getting half of the donations but said they were too expensive. She told KGTV that she has started her own GoFundMe to raise money.
Mr Gutierrez plans to spend the donations on helping him study and fulfil his dream of becoming an athletic trainer.
He told the Tribune Ms Gilles wasn't the first person to enter Starbucks without wearing a face mask.
“I would say about 80 percent of the people are cooperative and wear masks,” he said.
“Another 10 percent just forgot and are apologetic about not having one. And then there are maybe 10 percent who just don’t want to wear one.”
Governors in several US states have ordered requirements for face masks and imposed further limits on operations of bars and restaurants as the number of cases in the US continued to surge on Wednesday.