Matt Pliska, who has also survived leukaemia, asked his best friend Mikenzy Snell to be his prom date last year.
And when they attended the event in Waupaca, Michigan, U.S., they held up an emotional sign which read: "Real friends don't count chromosomes."
The message referred to the fact that Down's syndrome occurs when an individual has an extra copy of chromosome 21.
Mikenzy told WBAY: "I just feel like it really shows our friendship to the next level of where it actually is but people might not see behind closed doors."
Since TV news first shared the heartwarming picture on Facebook on March 30, more than 15,000 people have liked the image.
After Matt asked Mikenzy to be his date, she did everything she could to make sure that he had a fantastic night.
Mikenzy's mum Carmen told Yahoo Parenting that some of her daughter's 'best times' have been with Matt.
She said: "They see movies, go out for dinner and go to watch high school sporting events together."
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Matt and Mikenzy first became fast friends in fifth grade after they were partnered together during a class trip.
Mikenzy said: "He's such a positive person. He always has a smile on his face. I think it'll be a forever friendship."
Matt said about Mikenzy: "Honestly, I love [her]."
Mikenzy, who coaches Special Olympics basketball, said she hopes to one day have a career working with disabled kids.
Matt, meanwhile, is the team manager for both the boys' hockey team and the girls' soccer team.
His mum Flonnie said: "When you have a child that has a disability, they tell you they're not going to do this, you're not going to do that. Then you turn around and you watch when they're growing up."