what should I do when single track gets narrow
Presuming you cannot see the other rider before entering the narrow section (if you can, wait for them to pass through), going back is not my first choice - as others have commented, if there's enough room to turn a bike around, there's enough room to get past - even if it means both dismounting. 

Given most riders are human and can communicate even if they speak a different language, some form of dialogue should quickly come to a solution that meets the needs of the specific situation and individuals involved. Its also rare for very narrow sections to last for more than a few yards - perhaps "backing up" to a passing place would be quicker and easier. 

If there is not enough room for the bikes to to be passed around, and its too far to back up.... you could offer to swap bikes and meet up next week, same place, same time...... Especially if his bike is nicer than yours.