In fiction writing, grammar can be fast and loose. It's not as though there's a grading board that must rubber-stamp a book before it's allowed on shelves. More or less anything goes - as long as it's easy to understand what's being said, and the use (or lack thereof) of grammar does not distract from the story.
So on the one hand, there's a long tradition of fiction writers occasionally inserting a grammatically incorrect sentence in order to get a particularly visceral or spicy moment to punch through the text. It seems to me that this might be what the writer you're reading is doing. Those short sentence fragments might be taking some of the more unsavory details in their world and couching them in a rebellious tone so that they read as uncouthly as they are.
But on the other hand, you've seen this enough that it doesn't sit well with you. If these sentence fragments are jumping out at you enough to prevent you from getting lost in the story, then grammar is being misused. A little bit of creative grammar goes a long way.
As you think about what to say to your friend, I think these are the two competing ideas you should keep in mind. It's up to you as the beta reader to decide which side you think the author's sentence fragments fall on.