Shadi is correct in this. Also, the phrase insinuates a betrayal. So anyone can "bail" on anyone, but it usually means that there was some expectation of loyalty based on friendship or a previous relationship. So usually, it is friends who bail on each other, but sometimes, it can be two individuals who have made solemn promises to do something...and one of them reneges on the promise. Both of these relationships allows the feeling of "betrayal" if one party does not fulfill what they promise to do.

It means friend A and friend B had plans or a meeting, and friend A was not there. Or, if they didn't have specific plans it could mean that friend A ignored friend B so much that friend B felt like they were not friends anymore. "To bail" can mean "to quit, run away, leave", usually in a negative way.