What are good choices to introduce someone to A/H?

Honestly I'm not sure I would inflict that abomination on my worst enemy.
I would. I can picture it now... my worst enemy tied to a chair, daring me to do my worst. I smile and push the button on a CD player, which promptly plays the audiobook version of the whole trilogy, one book after the other. By the end of it, he'll be sorry he messed with me!
Robert Sobel's For Want of a Nail, hands down. A detailed alternate history of North America into the 1960's (written in early 70's) based on the POD of Burgoyne winning at Saratoga.
I second the mention of For Want of a Nail by Robert Sobel; it's a classic, though it does come with the proviso that the reader needs to enjoy reading a lot of detail - it's written (deliberately) in the style of a history book which goes into such things as annual coal production of nations.

For a shorter introduction to AH, try He Walked Around the Horses by H. Beam Piper (free via Project Gutenberg), particularly for anyone with reasonable knowledge of real history of the late 18th / early 19th centuries.