A good friend was given a3 yr sentence, but she qualifies for shock probation and if approved will be released in 6 months. She asked me to write a letter that her lawyer will give to judge. I would appreciate any tips to help me,as I am not exactly sure what or how long the letter should be..

The length of the letter doesn't matter. The important thing is to be clear and persuasive, so as to convince the judge that probation will be effective for your friend.