Can't verify Phone Number issue

[MickeyFromPace]

I managed to get a new iPad setup for a friend of mine that's technically challenged. He had an old iPad mini and during the conversion I had to change his account password, setup a new passcode to access the iPad and setup a new AppleID password for him and during all of these changes his account is setup to send the confirmation text messages to his wife's phone not his. I went in and tried to add his phone number and when I did I got a message stating that "Cannot Verify Phone Number" This phone number is already in use with Another account. I'm thinking that perhaps the reason why confirmation texts are being sent to his wife's phone is because of this problem. That being said, can anyone explain how to find out what account is being used and how to fix this issue? My concern is that iMessanger now uses the owner's phone number and it appears to me that any messages would go to his wife's phone instead of his.