Using something that is a part of evidence for an active case can lead to legal issues against you which would include leaking case evidence as well as potentially tampering of the evidence. Nothing may come of it depending on what exactly is in the notes, however I am not a lawyer and the risk should still be noted.
Law is very, very guarded about the chain of custody in terms of case evidence. There is a process that police take from the time they declare it evidence to when they show it in court. They have it documented, sealed off, and locked away and each piece of evidence also has a sheet that tracks who was the last person to touch it and when. It is absolutely crucial that they keep this perfect as any defects in this chain of custody could cause the evidence to be thrown out and in turn ruin the case. A good defense lawyer can easily pick apart a case to get their guy off free in many ways. All he needs to do is prove that something did not go according to procedure and declare it a miss trial. If you are found to have been tampering with or releasing evidence to the case, you may be in legal trouble too.
The best advice is to talk to this person's lawyer or the court house. Ask them about this evidence and how you want to publish it as a book as you were given permission to do so and if in doing so, it would affect the case. The lawyer could be shady though and tell you to go ahead in hopes it messes something up so you do need to watch out for that too. For further information with regards to law and how this should be handled, please see our Law Stack Exchange.
In terms of your original question, assuming the case gets closed and you are able to use the writings without issues, there shouldn't be an issue as long as you still credit this person as the source of the writing. You may have to be careful with how you proceed though as this person may come at you in a lawsuit saying he was not compensated enough should this bring in decent cash. Whether you choose a pen name or not as your editor's name is up to you though.