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Joseph Gardiner was captain of Pyle RFC's under-16s team in Bridgend, South Wales.
Family friend Adam Brooke, whose son was Joseph's best friend, told Sky News he was "such a genuine, good kid, a nice kid".
"He had his whole life ahead of him," he said.
He was taken to hospital with a viral infection after he went to the doctor with a "shortness of breath".
"An hour before he was talking to my boy, he was literally saying he was coming home that night," Mr Brooke said.
"His dad said to me at 1.55pm Jo asked for a coffee, so his dad went down to get him a coffee while he went to the toilet but obviously while he went to the toilet he suffered a massive heart attack.
"He managed to get himself back to the bed but that's when it went completely downhill. His dad said he came back with a coffee to them pumping his boy's chest."
Medics have said Joseph had an underlying heart condition, Mr Brooke added.
"What they've said is there was an underlying one from birth but obviously nobody would have known about it because obviously he's never been ill," he said.
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Mr Brooke has set up an online fundraiser to raise money to help the family following Joseph's death.
"The support's been absolutely amazing," he said.
"The family, you know. It's to give them, so they're not going to have nothing to actually worry about now.
"They're going to have enough to pay for his funeral, they're going to have enough left over for a gravestone and there'd be enough left over then for a tribute or something to him."
In a tribute, Pyle RFC said Joseph was a "great young rugby player in the making" and "an absolute credit to his family who loved him very much".
"The good ones always get taken first. But he genuinely was one of the good ones, the most selfless kid you'd ever come across," Mr Brooke added.
"He never asked for nothing. He was just one of those kids, he genuinely was just happy with life."