I've been on both sides. I'm still on both sides. I suppose it depends upon the situation. Right now I'm allowing Kizzy to fledge, but at some point I might decide she should be grounded. Having a flighted bird means more training. I need to find Kizzy's treat food before I can proceed further. In the meantime, I'm trying to keep her as safe as possible. She's so much lighter and more aerodynamic than Gabby that I'm thinking she might do better if I use the aviator harness to train her to fly well and recall, even from heights. Even clipped, a stiff breeze might send her airborne. I will always use the Aviator Harness, but accidents happen. I'd rather have a chance of retrieving her.
When Gabby turned twenty, I tried to get him to fly. I also tried to introduce him to a harness. I did all the wrong things. Even if I'd done all the right things, I'm sure he would have been very resistant to a harness. I'm not saying it's impossible to teach an older bird, but it is difficult. I understand why Amy's person is reluctant.
As time passes, it becomes harder and harder to find someone in this area who can competently clip a bird's wings. During the final years of Gabby's life, he had to go unclipped and therefore miss out on a lot of fun things.

He could still visit my mother and enjoy rides in the car, but I never found anyone I really trusted to clip him. Poor fellow.
As a pet store employee, I did gain lots of experience clipping wings. I never hurt a bird; however, back then I always used the same clip. Now, I realize the clip should vary from bird to bird depending on their weight, build, temperament and wing colors. I think if I were able to clone myself, I could do a good job on Kizzy.
There is a new avian vet in the area. She treated Gabby until we decided he needed to be seen by Dr. Speer. If Kizzy needs to be clipped, I guess I'll get her an appointment to see this local vet. When she saw how I quickly and easily restrained Gabby, she said, "I'd hire you on the spot." I know she could use my help, but right now I associate her with Gabby's death, so I'm not ready to break the ice just yet.
I hope all goes well until Kizzy has completed fledging and weaning. At that point, I will reassess her needs.