前者不明确是几个朋友,后者明确是一个朋友;前者较重于强调friend,而后者较重于强调father.不过,有时候可以互换.如:
who is he?
--He is a friend of my father.=
--He is my father's friend.
这种情况只能用第二种.如:
whoes friend is he?
--he is a frined of my father's.
还有对于这个问句:who is he
回答可以是:A friend of my father's.省略其它部分,而第一句则不可以.