Lohan left the Promises clinic in Malibu on July 13 after receiving treatment for alcoholism. Love Island star Calum originally claimed that he would be joining the Mean Girls actress on the wagon and ditching his party lifestyle, because he was desperate to stay with her.
However, Best has now dumped Lindsay and is dating former Shipwrecked contestant Felicity Jaine.
A source told the Daily Star: "Calum called Lindsay on Friday and said that he was finding her post-rehab existence dull and that he didn't want to give up partying. Calum loves Lindsay. But he's simply not in the same place as her anymore."
Lohan revealed that she was looking forward to a "calm" life with Calum earlier this month.