The actress, 20, who stars in Quentin Tarantino’s Once Upon a Time in Hollywood, spoke to PEOPLE at San Diego Comic-Con while promoting the film.
“Once he had his heart attack, I made him go vegan,” she said. “That wasn’t so much advice, as it was, ‘You have to do this.’ I didn’t really give him a choice. There was no way that was not going to happen.”
Kevin, 48, suffered what is called The Widowmaker and lost 51 pounds to regain control of his health.
“Going vegan isn’t hard, but it’s definitely something you have to get used to, being super aware,” Harley said of her father’s lifestyle change. “I’ve helped him through it, I’ve held his hand. It’s much easier to go through it when you have somebody.”
She continued, “When I went vegan, my best friend went vegan right before me, so she held my hand, and now I’m holding my dad’s hand for it. I’m really happy I get to be there for him for this.”
Kevin told PEOPLE at the Los Angeles premiere of Once Upon a Time in Hollywood that he “never imagined I would go vegan,” but the diet is working for him despite “f—ing hat[ing] vegetables.”
“Given a choice between a f—ing kumquat or an eggplant and nothing, I’ll choose nothing,” he shared. “So as a vegan, you wind up intermittently fasting by virtue of the fact that you’re like, well, none of this s— sounds appetizing right about now. You don’t do a lot of passion eating or boredom eating.”
Harley is also starring in her father’s upcoming Jay and Silent Bob Reboot, alongside a slew of Hollywood stars such as Ben Affleck, Matt Damon, Jason Lee, Frankie Shaw and Shannon Elizabeth.
Their bond has helped Harley take on her own projects as well, such as her band, the tenth, and her directorial debut for their upcoming music video.
“He gives me advice on all fronts. I think we keep each other in check a lot for sure,” Harley said. “We’re really, really similar people, and we feel things really similarly. When one of us is really going through something, or really emotional, the other one knows exactly what to do, because we experience the same feelings.”
She added, “We always keep each other in check, or are always there for each other when something gets rough.”
Harley stars as Froggie, one of the Manson Family members opposite Dakota Fanning, Lena Dunham, Margaret Qualley and Austin Butler.
Once Upon a Time in Hollywood is in theaters now.