It isn’t easy to be a good critique partner. You have to be honest and yet stand fast when you hit a nerve and get blasted for it. It does happen.
Don’t forget to be encouraging as well as honest. Focus on the overall story and add spotlights about missed commas, over used words, dialogue, flow, spelling and grammar as an add-on (things they need to watch for). If you can write an overall impression and give not only constructive feedback but some real appreciation and compliments along the way you give the impression that it wasn’t all bad and that makes everyone happy. Well, at least they won’t be miserable.
Don’t dawdle and keep your writer in suspense or wondering if you are crtitiquing or copying. Give a prompt review and if you need more time be upfront about it. Or, don’t do the critique until you know you have the time for it.
Don’t be pressured into doing a critique if you really don’t want to. You can’t be fair if they’ve bullied you into it.
Don’t go overboard and make a book out of it. Don’t become fixated on detailing every least mistake. Set your mind on writing a paragraph or two and rather than trying to perfect them all at once.