The 26-year-old rapper was found dead of a suspected overdose on Friday.
“She completely broke down after she found out about Mac,” the source tells PEOPLE of Grande’s state. “She can’t believe he isn’t here anymore. He was such a special person to her. She is very, very sad. … This is a nightmare for her.”
Miller (real name: Malcolm James McCormick) was pronounced dead at 11:51 a.m. at his Studio City, California, home, the Los Angeles County Department of Medical Examiner-Coroner confirmed in a statement to PEOPLE. A cause of death is yet to be determined, but a source tells PEOPLE the rapper had gone into cardiac arrest after appearing to suffer a drug overdose.
RELATED VIDEO: Mac Miller Dead at 26 of Apparent Overdose
One day after Miller’s death, the “No Tears Left to Cry” singer honored him on her Instagram account, posting a black-and-white photo of the rapper without a caption.
Grande, 25, and Miller — who collaborated on her 2013 track “The Way” — took their romance public in September 2016; they split in late April after two years together.
The rapper was open about his battle with substance abuse, revealing producer Rick Rubin helped him get clean in 2015 in a previous Rolling Stone interview.
“This planet has a certain gravity, man — it rises in the morning, it sets at night, and I never really followed that,” he told the music magazine of how sobriety has changed him. “But now I go to sleep at night and wake up in the day, and that’s been a beautiful balance in my life. Along with not feeling like s—.”
Barely a week after news of Miller and Grande’s split broke in May, the rapper was arrested on drunk driving and hit-and-run charges; the same week, internet trolls attacked Grande, blaming her for Miller’s relapse.
Grande responded, revealing she and Miller were in a “toxic relationship” as he struggled with his demons.
“I am not a babysitter or a mother and no woman should feel that they need to be. I have cared for him and tried to support his sobriety & prayed for his balance for years (and always will of course) but shaming / blaming women for a man’s inability to keep his s— together is a very major problem,” she wrote.
“of course i didn’t share about how hard or scary it was while it was happening but it was. i will continue to pray from the bottom of my heart that he figures it all out and that any other woman in this position does as well.”
“I was in love with somebody. We were together for two years,” Miller told Zane Lowe during a Beats 1 interview on Apple Music. “We worked through good times, bad times, stress and everything else. And then it came to an end and we both moved on. And it’s that simple.”
Indeed, a source told PEOPLE on Saturday: “It honestly seemed he was over the Ariana situation.”
On Monday, Miller’s close friend Shane Powers opened up about the rapper’s death on his podcast, The Shane Show, explaining that when Miller and Grande were dating, she had a positive influence on him.
“They were very much in love and I have to say, she was incredible when he was first sobering up,” Powers said. “There could not have been anybody more supportive of him being sober than Ariana. I saw that, I was around it, I took phone calls from her. ‘How do I help? What do I do?’ This little girl was unbelievably involved and helpful to him being healthy.”