I have been collaborating with another person on a paper which is almost complete.
Today I checked emails and they asked if I could share the Overleaf with someone to check calculations. I did that assuming that they were doing this so that the person could help out and check some elementary but long calculations in the appendix and that it was easier this way than sending them a PDF.
However, I then noticed that they had been added to the list of authors without my consent and without discussing with me beforehand.
I looked the person up and they have one paper from 2022 as far as I can see and is a PhD student at the same university where the lead author did their PhD, so it has slight vibes that they are helping out a friend who needs a paper. I cannot see anything that this third person is contributing apart from checking some working.
I have emailed my coauthor saying ''I noticed that X has also been added as a coauthor to the paper, could you advise what their contribution will be to the work or is it only to check calculations in the appendix''? Hopefully, they will have some very good reason why this third person is necessary or will improve the article.
Is this the correct thing to do? I don't want to ''stir things up'', but I feel like there is some etiquette that you cannot just add someone as an author to double-check some calculations, especially as there is a risk of sneaking in gift authorship or using the article to help someone out who ''needs papers''.
Edit: They answered and said the person is an expert in the area and will help to explain the physical meaning of the figures, not just calculations in the appendix.