The 23-year-old singer returned to Boca Raton, Florida, on Tuesday, where Grande was spotted with 25-year-old Miller and her mother at the airport.
"When Ariana got off the plane, Mac greeted her on the tarmac with a long hug followed by a kiss," a source tells E! News. "Mac also gave Ariana's mom a hug. It was a very emotional reunion and they stood talking for a minute before getting into the car."
A source close to Grande tells ET that the former Nickelodeon star is "an absolute wreck" following the suicide bombing that left 22 people dead.
"She cares more about her fans than anything," the source says.
Grande tweeted about the attack on Monday. "Broken," she wrote. "From the bottom of my heart, I am so, so sorry. I don't have words."
CBS This Morning's Gayle King spoke off-camera with Grande's manager, Scooter Braun, at 1:30 a.m. on Tuesday about how the singer is doing. Braun said that Grande is "not good at all," and is "hysterical" and "devastated" over the news.