Attention Grammar and Punctuation Junkies

My brain is on hiatus to the extent that I cannot even multiply 17 by 5 without help right now, so please tell me which of the following is correct:

"I'm a friend of Mortimer's"

or

"I'm a friend of Mortimer"

No money riding on this one, just my pride and a chance to say, "Nyah nyah nyah, how dare you doubt me in such matters." (Although the doubter in question will read this entry in which I am obviously doubting myself.)

A bleak day for me. Nothing worrisome, just weeks and months of tiredness suddenly dragging me down. Perhaps tomorrow can be a day of rest.