And that led to a tweet from American singer Miley Cyrus.
Willar, 19, who was diagnosed with leukemia in October, has been receiving chemotherapy.
Willar's friends, once they realized she would miss the concert, tweeted Miller, and asked him to meet Willar while he was in St. John's.
Two days later, he replied to their tweets, saying "Let's make it happen, see you soon."
"She was so beside herself," said mother DaynaWillar when Miller came to Hayley's room.
"She just covered her face, and she cried, and she said, 'Oh my God, I don't believe Mac Miller is in my room.'"
He was just so natural and so genuine and down to earth."
Down to earth
Miller, 21, whose real name is Malcolm McCormick, spent an unpublicized hour visiting with Willar on the afternoon of Nov. 19.
Dayna Willar acknowledged that while the rapper's music is raunchy and explicit, he was a pleasant visitor.
"You would never believe that he was as down to earth as he is," said Dayna Willar.
Willar shared a photo of herself and Miller on social media, and Miller left her with a rapper's bling necklace as a gift.
"He said, 'This has been everywhere with me, but now I want you to have it,'" said Willar's mother.
Willar and Miller also talked about their favourite recording artists, and he mentioned that Miley Cyrus was interested in working with him.
"Hayley said, 'I love Miley, she's my idol,'" said Dayna Willar, "And he said 'Oh yeah.'"
"And the next day, she gets a tweet from Miley, as well as Ariana Grande, who's another singer that Hayley really enjoys."
Mom now a Mac Miller fan
Dayna Willar admitted that while she hadn't known much about rap music previously, "I am absolutely a Mac Miller fan now."
Several months earlier, Hayley Willar had asked for tickets to Miller's St. John's concert as an advance birthday present.
But about five weeks ago, she complained of being tired, and found bruises on her legs. After blood and bone marrow tests, Willar was diagnosed with leukemia.
Willar, whose leukemia prognosis is positive, has been feeling upbeat ever since the visit.
"Hayley will even tell you, it's the best birthday she's ever had," said her mother.
"For someone to have cancer and to feel lucky to be where she is right now, I think that says a lot about just who she is. She's never once let this get her down."