Question: how to transfer contacts when changing apple id?
Answer: Sign in to iCloud.com from a web browser on your computer. Go to Contacts > Select all (Mac: Commad+A / PC: CTRL+A) > Click the gear on the bottom left > Export vCard. Then sign out and back in with his new account > Contacts > Click the gear on the bottom left > Import vCard and navigate to the vCard you exported.