Reviews have always caused angst. A good review in the right place can propel a book/film/show into the stratosphere, and let's face it nobody likes a bad one however much they may claim to ignore them.
Time was when only professionals could review, but no longer. With customer feedback on all retail sites and Goodreads, suddenly everyone's a critic. And these days, with more people dipping a toe into self publishing, the question of eliciting good reviews raises its own ethical issues.
I recently encountered a situation where a SP author had entered into a reciprocal arrangement with another SP author to review each other's books. A problem arose when, having read the book, she felt unable to give more than two stars as she could clearly see its faults. "How do I review the book without hurting her feelings," she asked.
The trouble with this sort of arrangement is it can lead to a You Scratch My Back and I'll Scratch Yours scenario that devalues the whole reviewing process. I've seen people say that they no longer trust Amazon five star reviews because of it, which is a shame because hearing others' opinions can be very useful, but only if the opinions are genuine.
If you feel your hands are tied, because you naturally want the author to look favourably on your own book, there is no way you can give an honest review. Even if you try to soften the criticism with blander statements, what use is that to potential readers who the review is for? If you can't tell the truth, don't do it.
My friend decided not to review the book in the end, explaining her concerns to the author by email. I think this was the best course, and a lesson for the future to avoid such arrangements unless you have already read and enjoyed a book and could endorse it with total honesty.
The best way to get a five star review is to write a five star book.