As I understand it, the question is can he give the reviewers' comments to his colleague. Of course. It will help the authors to accept the critique. What may happen is that the colleague gets so excited about the study, that he experiments and writes more material for the paper. He can then be added to the author list at this late stage, by mutual agreement with all the authors..
Thank you all for your valuable comments and inputs. My question is about giving my manuscript to an expert in the field whom I trust to reply to the comments (and check the technical content of the paper) to get a better and expedited chance to be published. In many cases, he/she has done an amount and quality of work that any of the three authors can have done. From the trend of your answers, it seems that he/she should be a fourth author, but in limited cases - depending on his/her contribution and the nature of trust and interaction between the authors.
The researcher who helps in correcting the grammatical error is eligible to include in the acknowledgements. If he contributes scientifically, for e.g., providing a better explanation or correcting some other errors related to the main content of the paper, he can be considered for co-authorship also.
If the expert contributes to the content of the paper (e.g. adds something to the introduction, literature review, discussion or any other parts) and/or replies to the reviewers, then (s)he should become a co-author. If the expert only corrects the language, then this is not necessary. Journals allow to add authors during the review process, but you and your co-authors have to discuss if the expert will be a 4th, 3rd, 2nd or 1st author depending on her/his contribution (if this was something relatively minor or something that completely transformed the paper: e.g. if the expert completely rewrote the literature review and the results part and added new data).