I am working on my PhD now for 1.5 years and was recently asked by my supervisor to assist him with a review. The reviewed paper was also written by a non-native speaker.
For my first publication I bought a book on English writing skills for research papers, which was very helpful for me.
Can I suggest this book to the author of the paper I got for review, so he can improve his English? I do not want to critize him, only to give him an advice I also give my colleagues. I am not sure if this exceeds my responsibility.