possessive with determiners (a friend of mine, etc)

We cannot put a possessive together with another determiner before a noun. We can say my friend, Ann’s friend, a friend or that friend, but not a my friend or that Ann’s friend.

    [determiner + noun + of + possessive]
    That policeman is a friend of mine.
    Here’s that friend of yours
    I met another boyfriend of Lucy’s yesterday.
    He’s a cousin of the Queen’s
    Have you heard this new idea of the boss’s?