You mentioned in your first post that this prisoner is maintaining his innocence and is currently "trying to free himself." A book about his prison experiences - which no doubt could make for an interesting book - will not work toward this goal. The book can, however, show why he is who he is, and why he is where he is.
You said you would "change the name," and I am not clear what you mean. The changing of names of
individuals in a true story will do little to disguise these individuals. There are only a few people who exist in a person's real life who will interact with that person. Real names can be used if they are used carefully. All that is said about these
identifiable individuals (even when they are not identified by name) should be true or pure opinion, to better avoid the risk of a lawsuit.
I think the story the prisoner has to tell could be a good one. I think many people may have a bit of a glorified picture of the life a prisoner leads (or an unrealistic picture in their heads of constant prison riots and rapes, or prisoners making friends with mice or birds
).