To give everybody a rest from Henry James:

An ESL student in my class wrote, "Now I am a friend of him."  Is there a
good reason I can give him for why it should be "of HIS"?

It gets funny with proper names, too.  If we knew each other well, I might
describe myself as a friend of Bill Clinton's.  But, at least for the
first four or five days of his term, I would have described myself as a
supporter of Bill Clinton.  Go figure.  Please.