The Bachelor In Paradise star spoke exclusively to New Idea about the moment he caught the blonde reality star cheating on him...with his best friend!
"I don’t understand why she would do something like that," a confused Grant says.
"She definitely had me fooled, that’s for sure."
Ali previously claimed that her American lover had only dated her for fame and she was left heartbroken after their steamy relationship came to an end.
However, Grant reveals it was actually him who called time on their relationship in March, after he caught her in a passionate tryst with his best friend – in his own home – just days after she landed in LA to start their new life together.
"The third or fourth day she got here, we went out and this night – when all this stuff happened – I had people over after going out to a club," Grant explains. "I live in a gated community so I have to go down and let people in sometimes.
"So, I leave my house with my friend Sabrina, to go and grab my friends and bring them back upstairs, and when we walk back inside, we come up the stairs and she is hooking up with this dude."
The former US Bachelorette and Bachelor In Paradise star says he then "kicked her out" of his home and said he never wants to see her again.
And as for his friend who was caught with Ali, Grant says they’re still the best of mates.
"I kicked her out and he slept on my couch that night," he says.
"The guy that this all went down with, he didn’t even know she was with me, he didn’t even know who she was.
"He thought she was some random girl at my house!"
In a series of texts obtained by the publication, it’s clear Ali begged her former flame for his forgiveness.
"She starts blowing up at me the next day saying: 'I’m so sorry. I was wasted. That’s not me.' And I was like, 'No, I don’t want to see you,'" Grant explains further.
"I told her to stop talking to me: 'I’m about to block you', so that’s what I did, I blocked her and she flew home the next day or something like that."
While Grant admits he was in love with Ali while filming Bachelor In Paradise in Fiji, things quickly started to fall apart when filming finished.
"When I was on the show I was [in love with her] but then after, when I hung out with her, I saw some red flags," he spills. "I went out with her a couple of times and we were drinking and she was just a little bit crazy.
"So, I wanted to make sure that she could handle the lifestyle that I live in LA because she wanted to get a job here and move here."
Unable to return to the dating scene since he was so badly burned by Ali, a devastated Grant warns Australia, "there’s your Bachelorette".
"That is literally one of the most disrespectful things you can do," he says.
"I don’t even know how to get past that level of disrespect. I’m still shocked by it.
"If that happened in my own house, I don’t know what was happening halfway around the world!"
The announcement of Ali as Bachelorette was met with outrage by viewers who claim they won’t be tuning in to watch her search for love... again.
"Not even worth watching," one fan wrote.