We have a 2008 Hyundai Santa Fe. Bought it used six years ago and after having it for a few months the gas gauge stopped working. We were told that the fuel pump wasn't working. We lived with it knowing that it would cost a lot of money to fix. Finally, had a mechanic replace the fuel pump. Apparently, the Santa Fe has two fuel pumps. So, of course, the gas gauge is still not working because the wrong one was replaced. Cost of repair was over $900. The mechanic doesn't care. So my questions are: Wouldn't a mechanic see that there are two fuel pumps when they take things apart to get to the fuel pump that they replaced? Also, do we have no recourse after paying for a fix that didn't fix the problem.
You have a valid reason to be angry. Actually, there's a couple of reasons.
#1 You should not have been sold a new fuel pump just because a fuel gauge sender was broken.
#2 The mechanic should have scanned for a code to identify which sender was defective.
He would have seen a fault code like: "Fuel Level Sensor 'A' Circuit...." or, " Fuel Level Sensor 'B' Circuit....." which would have identified which fuel gauge sender was broken.
Telling you that you needed a new fuel pump was a lie.
That said, there's a couple of ways you can look at this and it depends on if you plan on keeping your Santa Fe for years to come.
You just spent $900 bucks to get your fuel gauge working again and the d*mn thing still doesn't work. Plus, you were sold a fuel pump that you didn't need.
You can rightfully be angry and pursue financial satisfaction, or you can approach it with the attitude that both fuel gauge senders are identical, that they are notorious for failing in your vehicle, 1 has failed already and you have a new fuel pump and 1 new fuel gauge sender. (even though you didn't need them, yet.)
Personally, having a "glass half full" attitude is easier on my ulcer.
At this point if you want your fuel gauge to work you have to replace the other sender.
Your fuel pump/senders are located under the back seats. One sender is under the driver side back seat and the other is located under the passenger side back seat.
Frankly, replacing that other sender is an easy $50 (part) repair. (with an aftermarket sender). Plenty of youtubes on how to both remove the bolts holding in each back seat and replacing each fuel gauge sender on your Santa Fe.
(Like I said, these fuel gauge senders are notorious for failing on your car.)
At this point if you want your fuel gauge to work again you have to replace the other sender.
So now it's just a question of, if you're a guy who doesn't mind turning a few bolts to save some money.
The most time consuming part is removing and then reinstalling the back seat and that's the majority of the time/labor cost to just replace the fuel gauge sender.
Even if you are uncomfortable with opening the top of the fuel tank to replace the other sender, if you could present your Santa Fe to another repair shop with the back seat already removed, you could mitigate most of the labor cost to get the other sender replaced.
Otherwise, call around for some quotes. You needed a decent mechanic to correctly diagnose and repair the problem. (it's a shame that didn't happen)
Now that you know which fuel gauge sender needs replacing, any franchise repair shop employing low skilled wrench-monkeys can do it.
Just tell them you want the sender and the O-ring replaced and you want the old parts (to keep them honest on the O-ring replacement)

You've presented a detailed narrative to ground a fraud case. Impressive, and a real service to this person.
If you can get proper documentation from a respected mechanic (diagnosis, photos, etc.) you could sue in small claims court for damages. But that will take time. And, like below, you'll need evidence.
If you paid with a credit card, you could put the charge up for dispute. (Be prepared to submit evidence of incompetence or fraud).
You could call your local news station and ask them to intervene. But only do so after you have a consistent, verifiable, evidence-based narrative.
As I know fuel gauge and fuel pump are two independent units that do not affect each other.
One of Scotty's videos about that
https://www.youtube.com/watch?v=w7VjqfzMiTo