the term refers to one that serves as a drinking companion, specifically to a bar.
a bartner can be a friend, relative, classmate, coworker, etc.
an ideal bartner would be one who enjoys a similar drinking environment, holds a similar (or superior) alcohol tolerance, has similar taste in the opposite sex, is skilled at acquiring rides, etc.
After their microbiology exam, Pete and Tom bumped into each other at the local bar. From that day on, they were bartners.
Although Lisa and Tina had little to talk about at work, they were the best of bartners once Friday