Which author's country decides whether a waiver applies?
It depends on the publisher, and the differences are large. Wiley, Taylor & Francis, Springer Nature, Sage, Oxford and Cambridge read the corresponding author's affiliation. Elsevier requires every author to be in the same Research4Life group. IEEE applies the lowest entitlement in the group and gives nothing if one author is ineligible. BMJ additionally asks where the research was carried out and whether it was funded.
Detail
The practical consequence
Naming the corresponding author is a financial decision. A co-author abroad in that role can remove the reduction entirely, even when the work and every other author sit in an eligible country.
Mixed teams
At Elsevier a mix of Group A and Group B authors brings a full waiver down to 50%. At IEEE a single author in an ineligible country removes the discount altogether rather than reducing it.