How can I oppose my advisor granting gift authorship to a collaborator?
I would start by asking your professor, in a non-confrontational way, why he did this. Perhaps there is some reason that is not obvious to us. Maybe he felt that the paper had a better chance of being published if this third person was involved. (Not saying that he was right to do this, just that he may have had some reason.) You might say something like this:


  Professor X, as you know, I'm pretty new to academic publishing, and I'm trying to understand the process better. I was under the impression that only people who contributed to the research should be listed as authors. I'm curious why Joe Smith was added as an author. Did he make a contribution I don't know about?


If you think you may publish more papers with this professor, discuss your expectations about authorship with him and come to an agreement.

EDIT: Several people have suggested that the phrasing above is still too confrontational. I think they have a point, but I'm not sure we can come up with a phrasing that everyone is happy with. So I suggest that the OP read the comments below for ideas on how to phrase this.