Here’s an exercise you can do the next time you go to the bookstore: research your favorite writer’s whole catalogue and figure out their worst book.
Go buy that book. Read it. Cringe through it. Note its virtues.
Now ask yourself if you can produce something at least as good as this. Chances are that you can.
With Self-Publishing allowing for anyone to enter the arena of fiction writing, there are no standards in place that would prohibit inferior works from reaching readers. Books are often error-laden and stiffly, quickly-written and lean more on heavy marketing than quality of the product. I’d advise that you be honest when reviewing their work– or if they ask for a review, be polite and decline.
But also note that you can publish something better than this. You can correct the market quality by the taking the time and thoughtful care to go over your book many times. Have friends and family review them with you if you can’t afford an editor. (I can’t. And errors keep popping up even years later.)
Make it the best damn thing it can be because even if it doesn’t make a lot of money, it can outpace at least half what’s out there, or, at the very, very least, it can’t do worse.
And that’s still a success.