You know that saying, "Can't see the forest for the trees?" Well that's what happens to a writer when they spend months--even years, writing a novel. The writer is so invested in the story that they've lost all objectivity. And that's why they seek out critiquers--for honest feedback about what works and what doesn't. So if you've signed up to be a critiquer, please be honest because there's nothing worse than giving your writer the idea that they've written the greatest novel of all-time when it's obvious that it needs work. Why? Because most writers are trying to get published and the last thing they want is to submit a manuscript to an agent or editor that isn't ready.
As a critiquer, your job is to add water to this picture because, as you can see, when left to their own devices, writers can't recognize things that need improvement. Even when it's glaringly obvious to others.
So my best suggestion is this: If you really care about the person whose work you're critiquing, keep in mind that if you don't find something that could be improved, you're failing at your job. Remember, they're coming to you specifically for ways their manuscript can be improved, so please, be honest.