itunes account on husbands phone

My husband got a new 6s plus. I used itunes to transfer his content from his old 6s to his new 6s plus. everything was fine until i tried to download the facebook app to his new phone and the phone asked for MY itunes account password. I used my husbands apple id to log on to itunes when I transferred his content. He does not have a separate itunes account. everything else on the phone is associated with his gmail account.