The actress, who also appeared in her uncle Robert Rodriguez's Grindhouse movies, pleaded with Fox to spare her pal from another spell behind bars.
In the letter, which was also signed by other Lohan friends, Avellan wrote, "I have had the pleasure to get to know her (Lohan) very well and have endured the past year with her through her highs and lows.
"I am desperate to get this message to you. She is working really hard to be better and has done everything possible to stay healthy and clean. We (family members and loving friends) have separated her from anyone who can harm her or deviate her from her goals which are to be a model citizen.
"She is aware of her impact in this world and wants to rise from this. A few days ago she had a very low moment and made a mistake that has her so terrified it is breaking my heart. I am truly concerned for her well being."
But Avellan's letter, which has been obtained by TMZ.com, failed to keep Lohan from jail - Judge Fox sentenced her to 30 days in custody without bail for violating the terms of her freedom.
The actress, who spent 13 days behind bars and another 23 days in a lock-down rehab centre this summer (10), failed mandatory drug tests earlier this month (Sep10).