That boyfriend of Jane's is really a coward. （正确。of Jane's 是双重所有格。在“指示代词+名词”后面的所有格应该用双重所有格。）
That boyfriend of Jane is really a coward. （错误）
He is a friend of my father's. (相当于 He is one of my father's friends.)
He is a friend of my father. (相当于 He is friendly to my father.也可以表示：他是我父亲的一个朋友。)
