well, to be honest, how well do you know another person? you admit yourself you havent seen him for two years (if he has ben here, and you have been away) He could have changed, he could have got into drugs, he could have admitted it, he could have been provoked.
I think you only get a retrail if the conviction was unsafe, or if due legal process wasnt served. You might be thinking of an appeal? ....
As to what the lawyer can talk about with his parents, i would assume it is up t your friend how much the lawyer discloses to a 3rd party, as he might not want it discussed with his parents,