1. Bar Tools.
If your friend is just starting out, they will be delighted when you present them with their first muddler or citrus press. Even if they already have a full set of tools, if they're often in the habit of making more than one drink at a time, they could probably use more. Things that are always welcome: shakers, hawthorne strainers.
2. Exotic Liqueurs.
I love it when friends who are traveling bring back exotic liqueurs from other countries (or states) that I can't find locally. It's a great gift because it doesn't have to be very expensive, it shows they were thinking of me, and I get to experiment with something I've never tried before.
3. Cocktail Books.
If your friend is just beginning to delve into the delicious world of mixed drinks, I suggest Gary Regan's Joy of Mixology, which covers pretty much every aspect of the craft, or Ted Haigh's Vintage Spirits and Forgotten Cocktails, with recipes for 100 classic cocktails. If your friend has been at this for a while, try The PDT Cocktail Book or Left Coast Libations.
4. A Vintage Ice Crusher.
Currently I'm making crushed ice the old-fashioned way, by beating the heck out of it with a rolling pin, but I imagine that crushing ice for a mojito or mint julep with one of these vintage ice crushers would be delightfully civilized.
It's amazing how just a few drops of bitters can completely change the taste of a cocktail. I guarantee your friend will completely geek out over this set of 12 different kinds of cocktail bitters, including flavors like celery and rhubarb. If you don't want to completely overwhelm them, try the 6-bottle set, featuring relatively tame varieties such as mint, grapefruit, and peach.
6. Vintage Glassware.
Delicious drinks look even more delicious when served in lovely vintage cocktail glasses. Bonus: vintage glasses are often smaller than modern versions, which makes them better suited to the smaller portions of vintage cocktails. Try a search on ebay or Etsy, or just run down to your local thrift store. I have a set of vintage champagne coupes that I picked up at the Salvation Army for only 15 cents apiece.