In my research group, there is a researcher who usually approaches other PhD students who are about to write papers in order to offer his help. Normally, he does not actually do any research related to the paper. Instead, he reads our papers, makes comments about the writing style and sometime re-writes some sections in order to make them more readable. At the end, he will claim the co-authorship of the paper. I can see it is very helpful for new PhD students who don't have much experience in writing papers or articles. However, it is sometime uncomfortable for me to see someone who doesn't do actual research but still manages to get an authorship.
So, I want to ask if it is a common practice for someone to help writing a paper without doing any actual research relating to it, and claim the co-authorship. If it is not, how should I react if someone wants to do the same to me?
