Thank you for the information and your question. I am sorry to read about your wife's situation. I am not sure that the detective communicated very effectively since what you relay that they said is not a reason not to take the case. Perhaps what the detective meant to say was that after speaking to the person your wife says molested her and your wife, they don't believe they have a strong enough case to make probable cause
to arrest him. That may be because he had someone else who could vouch for him or because your wife couldn't provide detailed information. That, I can't second guess, I can only speak to the general reason why the police would not want to go forward on a case of child molestation from many years before.
That said, your wife can go to the prosecutor for your County and file a report/complaint
directly with them for the molestation. The prosecutor's office has their own investigators and they can determine if they think they have a strong enough case to amount to probable cause to arrest, which is the legal requirement. If so, then they might go forward with her case. Ultimately though, in our criminal justice
system the State (prosecutor) has prosecutorial discretion in all cases. The only way to put pressure on them if they don't want to go forward is to go to the press or your local State representative and complain that they aren't doing their job. They still have the final say, but that could put more pressure on them.
Depending on how old your wife is, she may or may not be able to file civil suit in tort against this man. Unfortunately Arizona does not have a special statute of limitations yet on the civil side for child sexual abuse, so the regular statute of limitations runs. That SOL is 2 years. However, it doesn't start running until the victim is 18. So, if she is 20 years old or younger, then she could find a local personal injury attorney to file a civil suit against this person for the molestation.
Those though are the only options. Please let me know if you need any clarification. I would be glad to assist you further if I can.