Scooter Braun said he and the singer met 19 families who had lost loved ones, which was the "hardest two hours of either of our lives".
Ariana suffered from trauma after the attack and questioned whether she could sing her songs ever again, he added.
But she decided to do a tribute concert so the victims didn't "die in vain".
"The terrorist made a mistake... they picked the wrong God damn show," Scooter said of the bombing on 22 May - in which 22 people died.
"Because if they thought we were going to roll over they don't know Ariana and they don't know me."