Urk!?
I'm of the opinion that no ethical vet would subject an animal to anesthesia twice for an operation, which can be performed all at once.
This is very strange...
Possibly they did not understand what their vet said. No vet would remove one testicle and insist on a return visit for the other one. Some vets might insist on a return visit to be sure Felix is healing properly (though this is not common for neutering, it is for spaying, since they take the stitches out on the followup visit). I don't think your friend got the correct story.
Thanks for the feedback. I also called my vet who said they believe the practice would be considered unethical.
Unfortunately, this isn't a friend. I'm an active member of a cat forum where I provide advice and resources for people who conduct Trap-Neuter-Return (or have strays/ferals show up in their yards and what to know what to do). According to this person she has 66 cats, and I've been encouraging her to get them spayed/neutered. I suspect someone mentally unbalanced and potentially a hoarder - or just someone attention-seeking. Hard to know. I alerted the board moderators, but there's apparently not much they can do.
Some people are just unbelievable.
She does sound like she has a problem. But, maybe she didn't understand the vet. It's possible the cat had cryptorchidism, where one or both testicles fail to desend. In that case, it's very possible he may not have been completely neutered the first time.
Sadly, it's worse than I thought. :( She's breeding DSH cats "because of their beautiful coats and delightful appearances." She states that she believes removing one testicle reduces the risk of "making too many kittens at a time." I can't imagine there's a vet in the world that would do this as this makes no medical sense. I don't know what to think, but there's nothing that can be done.
Oh dear, maybe you need to make a phone call.