Contractions ("can't", "don't") in a mathematical paper I am refereeing a  paper (a good one) for a mathematical journal. The author repeatedly writes like this: "we can't do", "we don't know" etc. He is not a native English speaker, and this paper is his first paper in English.

Question. I am going to ask the author to change "can't" and "don't" to more formal "cannot" and "do not". Is that a right thing to do? 

I am not a native English speaker either....

(Since a referee is supposed not to disclose their identity, I ask the question anonymously.)
My experience has typically been that the more formal mode, with no contractions, is preferred for most scientific publications.  It is not a terribly strong or important custom, however, and in one of my more high-profile multi-disciplinary publications, I actually found the copyeditor introducing contractions into my writing!

In short: worth mentioning, but not a big deal.  As a reviewer, you might say something like: "I found the contractions distracting, and would advise removing them" but I wouldn't recommend being much stronger than that in your statements.