Answer
What happens if one of my co-authors is in a high-income country?
It depends entirely on the publisher. At Wiley, Taylor & Francis, Springer Nature, Sage, Oxford and Cambridge nothing changes, because only the corresponding author's affiliation is read. At Elsevier a mixed team drops from a full waiver to 50%. At IEEE a single ineligible author removes the discount completely. At BMJ all authors must be in a listed country.
Detail
Before you finalise the author list
If you are collaborating internationally and cost matters, check the publisher's attribution rule before submission rather than after acceptance. The rule is usually stated on the same page as the country list.