Working as a sensitivity reader or a beta reader for a writer friend is one of the greatest joys of being part of the writing community. You get to read a new story before anyone else and you have the privilege of helping your writer friend turn their just-pulled-from-the-cave-wall stone into a highly polished, beautifully cut, sparkling diamond.
Obviously there are no hard-and-fast rules about what you should and should not be doing as a critique partner (beyond Wheaton’s Law). You and your author will find your own communication style and rhythm, as well as levels of honesty and helpfulness that you’re both comfortable with. However, the whole point of stepping up as a critique partner is to support your writer friend and help them make the book they’ve written the best version of itself that it can be.
Sometimes this means you have to point out flaws, but it also means that you should be pointing out the stuff that’s good, that really works for you, and connects with you emotionally as a reader. Writers need to know not just what needs to change, but also what needs to stay the same.