It could be a tricky situation. Obviously the politics within a research group (or between research groups) aren't your responsibility to manage, but I would assume that the friend is one of several authors. While everyone who has authorship on a paper has certain ethical responsibilities, in practice it's complicated.
Idk how much of this you already know, it sounds like you're pretty familiar with academia? So I might be telling you stuff you already know.

Idk the field that this paper is in or what the norms are, but like in my field I could be an author on a paper because I handled one part of the work but not have had anything to do with other parts, and in terms of authority and influence it could be tricky and delicate to try to tell a more senior researcher that there was an issue with part of their paper that was their responsibility.
The concern might be heard more effectively if taken directly to the person who's considered to have the most general/overall responsibility and control, either the corresponding author or the first author