We form the 'double genitive' with
noun + of + possessive pronoun mine/yours/his/hers/ours/yours/theirs
Zelfstandig nw + of + mine/yours/his/hers/ours/yours/theirs
A friend of his
OR
zelfst nw + of + my/your/his/her/our/your/their + zelfst nw + possessive 's
A friend of my brother's