See if he can recover the ID/password. Use every email address he can think of.
Apple ID - How to find your Apple - An iCloud e-mail address is an Apple ID.
Apple ID - Find -
If he forgot the Apple ID (iCloud) password, enter the Apple (iCloud) ID, and click Forgot Password. See if he can get a Rescue or Notification email option if necessary. Link to contact Apple Support at the end of the 2nd article.
Apple ID - Manage -
Apple ID – Forgot Password *
What you do with the old phone depends on what you plan to do with the old phone. If you plan to sell the old phone or have someone else use the old phone, do the steps in the article below.
Selling/Giving Away Device.
If you aren't going to do anything else with the old phone, you can leave it as it is.