"Sex became a part of Lindsay's recovery. We didn't get out of bed for days. If you have an addictive personality like Lindsay you need to replace the high you got from drugs."
Riley also claims Lindsay was in love with him and the pair planned to buy a house and settle down.
The 24-year-old professional snowboarder said: "Lindsay would tell me that she loved me and I would say that to her too. She told me she had only ever had three serious boyfriends and I was one of them.
"We even planned to get a house and live together in Utah. We were together every day for a month and I could tell she loved me by the way she cried when we said goodbye at the airport."
The relationship ended soon after Lindsay, 21, returned to Los Angeles in November and threw herself back into her hectic movie career.
Riley, who claims he still speaks to Lindsay every day, added to Britain's News of the World newspaper: "Her whole attitude changed when she got back to Los Angeles. She is surrounded by movie stars and does whatever she wants. There are a lot of fake people. I don't think it is healthy."
Since the split, Lindsay has been romantically linked to Paris Hilton's former lover Stavros Niarchos, Heath Ledger and a man only known as 'Spencer'.
Lindsay is said to have dumped Riley because he was jealous of her fame.