Sad story, but you haven't asked a true question, other than "how is this legal?" If what you say is true then it's not, but that's why we have a legal system. Some times it works some times not; but it's the best in the world.
Arizona is a no-fault state, so the issue of adultery isn't a legal issue that the court can even consider. Your question seems to be more related to his mental health treatment than a family law question.
Your friend may be elligible for what we call in Illinois an Order of Protection due to his protected status as a disabled person. I hope he is getting the counselling he needs. As to the adulterous party issues a private investigator can run some low level social media searches to try and find out if she or her "friends" have provided public evidence of their "love". More information would help as to how she cut him off from his child. As I am often in Court have him call my assistant Dan London at 312807-3990 to set up a private telephone consultation. At the consultation we can discuss the confidential details we need to craft a strategic plan. These details should not be broadcast over the Internet.
This is a general answer and does not address the specifics of your individual case. To give the specific answer you need our firm needs you to come in for a consultation.