Dyer said the letter from Dani made him reflect on the pain he'd caused his family and commit to continuing his treatment at the facility in South Africa.
Dani Dyer shot to fame on Love Island, winning the 2018 series. She and her father have since appeared together on Celebrity Gogglebox, an Italian travel series, and on their podcast Sorted With The Dyers.
Danny Dyer, who shot to fame in the 1999 film Human Traffic, has gone on to become one of the most recognisable stars of British TV and film.
In his interview, the 47-year-old spoke about how fame has affected his personal life, leading him to make "a lot of bad decisions".
He said at one stage of his career, he was "doing a lot of drugs".
"I'd go to nightclubs and DJ sometimes but in general just wave off of balconies and I'd get paid quite a lot of money to do it," he added.
Dyer says looking back is "awful" and that his days of "hedonism" and "mad behaviour" are over.
He said one of his turning points came in 2013, when he was offered the role of Queen Vic landlord Mick Carter in EastEnders, which he says he took because he "had no money".
"I had bailiffs [coming round], no-one would hire me and the more I'm doing these club appearances, the more drugs I'm taking, and drink, so I'm in this weird spiral."
Four years later, he made the decision to go to rehab in Cape Town during an awards show after-party at his house.
"I remember I had this moment where I was sitting in my ensuite trying to work out how to put a pair of jeans on - I was that off my head.
"I looked up, I looked at my wife and I could just see how tired she looked and I could hear kids running around downstairs, and I thought 'I need to sort my life out'," he added.
Dyer says it made him realise how addiction "affects so many people around you, not just yourself".