Whether we use "come back" or "go back" usually has to do with position and perspective. The general rule is that we use "go back" when there is movement away from the speaker and "come back" when there is movement toward the speaker. However I have encountered a situation where the rule does not really work. Here is a bit of context, I am in the UK and my friend is currently abroad for a week. When I ask her "Have you done xxx yet?" She answers "I will do it when I go back". Even though my friend is abroad, I'd expect her to say "I'll do it when I come back".
Neither of us is a native speaker so I'd like to understand what form is correct and why.
Another situation that I can think of is the followong one:
I text my flatmate: "hey mate, you forgot your wallet at home"
I'd expect him to reply "Thanks, I am coming back!" even though the movement is away from the speaker (i.e. my flatmate). However, he'd definitely call his boss to say "I'll be late because I have to go back home to take my wallet that I forgot".
I am unable to explain to my friend when "come back" or "go back" should be used. Could anyone please help?