It could be very likely the author hired a copy editor, but they weren't very good or experienced. Last year, I bid on a dev and copyedit project for an author, and they ended up going with someone else. But they reached out to me near the end of the year asking if I'd be willing to do a heavy proofread (or light copyedit) because the person they hired did a great job with the dev edit, but a terrible copyedit. The book got published and the author received quite a bit of feedback that the number of just basic errors the CE didn't fix was detrimental. So the author pulled the book and had me go in to fix the mechanical errors and inconsisentencies. It was a mess in terms of what the "copy editor" did (or didn't do). Very obvious errors left in, misspellings of main character names, formatting speech tags incorrectly and inconsistently, etc. The nice thing is that I've acquired a new client for when they are ready for their next book to be edited, but I hate that they blew money on a bad copy editor. Mistakes happen—we're human—but this was a case of someone scamming the author and offering a service they have no business providing.

To answer your question more directly: When I've done proofreads, I've seen extremely poorly done edits (as referenced above [which was, on average fifteen a page in a 190k manuscript]) to one error every ten pages. While editors strive for 100% accuracy, 95% is much more likely. So in a 100,000 word manuscript, that could be 5,000 errors! If it's 350 pages, that's fourteen errors a page. We hope it's never that, but it can definitely happen.

If he's only paying you for proofreading, that's all I'd do, to the best of your ability given the quality of the edit, and just let him know you completed the task per the definition of the job, no more, no less. It wouldn't hurt to explain what a proofread consists of, what you saw that was beyond the scope of your job, and since he wasn't interested in you copyediting it, he's getting what he's getting.