Double meaning?
In this case, the meaning is ambiguous and could mean either of the two you suggested as nothing in the phrase specifies who will be making it. I originally read it as she was leaving to make the coffee, but both are equally plausible. Is there any extra context around it that would clarify? E.g. are they in a location where making coffee is possible?
"She left to make the coffee" would be a considerably less ambiguous way of putting it!
I'm not sure we should really look to Jeffrey Archer as a model of literary style and clarity, you know... ;D