Come back vs Go back

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 in Italy 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".

Could someone explain why she can't use "go back" in this instance.

Another situation that I can think of is the followong one:

I text my flatmate: "hey mate, you forgot your wallet at home"

He'd reply "Thanks, I am coming back!". He would not reply "I am going back", would he? But 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".

Thanks for your help