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?