Extreme sports professional Riley, 24, met 21-year-old Lindsay while they were both having treatment for hard drug addiction at the Cirque Lodge centre in Utah.
The film star slipped the ski-slope hunk a secret note asking for his mobile phone number. Riley couldn`t resist and Lindsay was calling him up right away.
"From that moment it was on," he recalled as he described their first snatched passionate kiss in a corridor, strictly against the rules. "The chemistry between us was so strong we couldn`t help ourselves.
But once they left the Cirque Lodge- after 60 days on the programme-they moved into a romantic log cabin in the nearby mountains They locked themselves in and were at it like rabbits.
Riley revealed how Lindsay used orgasm after orgasm as a potent substitute for the huge chemical hits she was used to while partying.
He explained: "When you orgasm, your endorphins shoot up and it becomes a massive natural high. If you have an addictive personality like Lindsay you need that to replace the highs you got from taking drugs all the time. Sex became a key part of her recovery.
"And we didn`t get out of bed for days. It was the perfect place-roaring fires with amazing views over the Sundance ski resort.
"The first time we had sex I couldn`t believe I was looking down at Lindsay Lohan naked. We`d barely gotten through the door when we just ripped each other`s clothes off.
"Lindsay is so hot. She has a great body. Her backside is fantastic, perfect, all plump and round.
"She has great curves but her belly is nice and flat and toned. We couldn`t get enough of each other."
And Riley insisted the connection between the pair wasn`t just physical.
"Lindsay would tell me she loved me and I`d say that to her, too," he said. "And I don`t say that to a lot of girls.
"She told me she`d only ever had three serious boyfriends-and I`m one of them. We even planned on getting a house and living together out here in Utah. We were together every day for a month and after that I could tell she really loved me a lot by the way she cried and cried when we had to say goodbye at the airport."
But Riley revealed their relationship changed when Mean Girls star Lindsay got back to Los Angeles to pick up her career. And he confessed he is terrified she might return to her bad old ways.