A picture might help inform this discussion.
I personally see fixing a twisted fin as setting a broken arm. People on here make unfair comparisons to cutting off fingers and arms. If you don't cut bone/spine you are doing no such thing. Fingers don't grow back fins do FYI.
He does not like the way his fish looks when it swims because he believes his fish does not "use it very well." Cutting a fin is not necessary, since its just a cosmetic thing. But whatever, if he wants to cut it, its his choice. I just won't be sympathetic when he comes back posting a thread on how to cure his fish's rotting fin or something of the sort.