Its not about peoples choice as much as it being about fact. This is not an extreme case at all, ive seen pictures that of cases that are though that can literally make you sick.
I disagree with knowingly buying fish that will become too big for your tank. Not knowing means you probably should have bought the fish with some research but all aside, its something that is none of my business. If hes doesnt know the fish will outgrow the tank then he can be informed. Be if he knows and wants it anyway well then thats his choice and my nose over here minding my own business lol.
The doesnt lie so much with the swimming room as it does with other fish. Fish that dont have ample room become stressed and also become aggressive and territorial. Usually it ends with fish fighting and eventually death. Also because they become stressed they become more susceptible to disease, parasites, and infection from cuts and nipped fins. Just letting you know the facts.
Good luck with them and skim 24/7
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.