Yikes!!! I'm actually a bit confused--do they tape the flight feathers together to make the wings... not work? Or do they literally attempt to tape the wings down so the bird can't spread their wings???
Either way, taping is WAY worse than clipping. If they were going to take action to prevent the bird from flying, clipping would have been a better option. I am on your side, I fully believe that if someone adopts or purchases a bird they need to be prepared for bird behavior, up to and including... flying. The thing birds are most well known for doing. Lol. But, if you can't stop someone from owning a pet that they're absolutely ill-prepared to care for, and you know they're going to do something against the pet's best interests, then at that point it's just a matter of damage mitigation rather than prevention... After all, they aren't doing anything illegal and so you can't stop them from preventing the bird from flying. So this person should be encouraged to do a clip instead of taping. If they can be convinced to do a light clip, so that the bird can still fly but they can't fly as far or as fast, then that would be ideal. Otherwise, a clip that allows the bird to glide safely to the floor rather than drop like a rock is best.
I think working at a pet shelter was one of the most enlightening experiences of my life, although it was also very discouraging sometimes. Seeing the hopeless number of homeless cats and dogs out there really gave me some perspective. That experience doesn't 100% apply here, because I assume this family who's out there taping wings like absolute maniacs probably did not adopt but rather purchased this bird, but some of it does transfer... namely, if you take it upon yourself to feel responsible for every bird in the world being cared for to the same standards that you care for your babies, you will end up exhausted and heartbroken every time. Not everyone views their pets like family, so you need to bend in places that you don't want to bend. In this case, we may both agree that clipping wings is not the best thing for this bird's overall well-being and happiness, but there is probably nothing you can do to convince these people to keep their bird's wings intact. That's where clipping becomes the compromise that is best for the bird, and at the end of the day just remind yourself that you did what you could and don't let it weigh on your conscience.
So, I would simply tell them--ideally, birds should be able to fly, both for their mental health and for physical exercise, not to mention safety, but when I discouraged you from wing clipping I did not realize that taping would be your next best alternative. Taping is not good for the bird and so clipping would be better, and it should be done by a qualified vet tech or groomer.