Someone I know says, e.g. "my friend his book" instead of "my friend's book" or "the book of my friend", he stubbornly believes that the form with his/her is correct, but I can not find an answer or explanation on the internet to this. I learnt that the "long form" of "my friend's book", for example, is "the book of my friend", and I never heard the his/her story, alas it sounds very wrong to me. Can anyone explain what exactly is the right form and if the his/her is used in the english language in that context?