Ok- first off, let's not beat up on this guy/gal because they didn't quarantine. Personally, I have my doubts as to if quarantining fish from LFS is really as important as popular opinion claims it is (at least when it comes to preventing ich). Here's why:
1) Quarantining fish is not necessarily going to prevent white spot (or "Marine Ich" or Cryptocaryon irritans). Let's call it "Crypto." The organism that causes Crypto is virtually ubiquitous. It can come in on live rock, sand, anything at all that's ever been exposed to natural sea water. Even if you tried to quarantine everything, even live rock, you'd probably still get it in your tank at some point. Thus, the really issue isn't trying to keep the parasite out of your system (which is futile IMO), the real goal should be to keep your fish from becoming badly infected with the parasite. Some tangs, especially blue tangs and powder blue tangs, are like Crypto magnets. As soon as they get stressed, they get it. So I wouldn't freak out when one of them shows up with a few white spots. I would simply try to address the stress the fish is under, and take measures to reduce the population and impact of the parasite (such as use of a diatom filter to remove the form of the parasite that moves in the water column). Most important will be ameliorating the stress of the suffering fish.
2) Quarantining a fish may (or may not - depending on the circumstances) cause the fish more stress than it prevents. Fish get stressed when their environment changes. When you quarantine, you're forcing the fish to go through one more total environment change than it would if you didn't quarantine. Now, if your water parameters are very different than those of the LFS, then I'd say, yeah, it's probably a good idea to let the fish acclimate to the new water parameters before you introduce it to the other fish (i.e. let it deal with one shock at a time). But if your water is similar in salinity, temperature, pH, etc. as the LFS, I don't think quarantining is an advantage.
3) The fish that make it to your LFS have likely already been exposed to every pathogen, fish disease, and fish parasite that they're likely to be exposed to in the course of their captive lives. I don't know if it works the same way in South Africa as it does in the US, but in the US, fish arrive in the US first to a large distributor. They sit at the distributor for awhile before they get to the LFS. Thus, I'd say that most the "weak" fish, or diseased fish, have pretty much already died off by the time they get to your LFS. Now, I imagine that some fish might be "carriers" of diseases which don't do them much harm, but might do another species of fish harm. However, in that case, if the fish is a carrier, I don't see how quarantining the carrier fish is going to rid it of the disease or parasite anyway.
The bottom like is: IMO, whatever disease or parasite a new fish might have when it comes to the LFS, your current fish have likely either a) already been exposed to it, or b) are going to get exposed to it even if you quarantine the new fish.
To be clear, I'm not advocating against quarantining per se - I'm just saying that, when it comes to fish, and trying to prevent disease, I don't think quarantining necessarily makes much of a difference. It might in some circumstances, but in the case of presented by the OP, I don't think quarantining this tang would have necessarily prevented it from getting Crypto.
I do think that quarantining corals is a very good idea... but that's another story.