welcome to fishlore jjkoletar...I'm so sorry for the loss of your fish rip butteyfly!
but blaming our site for your loss is a bit harsh....we don't have as many salty members as we do freshwater which is unfortunate ...but researching a fish prior to buying it is just as important as relying on a site to help you out....I hope you have better experiences to come !
I just bought a large Heniochus as well. They are very hardy, and if it had ich, it probably had it when you bought it. The best way to prevent this is cleaner shrimp and quarantine. I have saved multiple expensive fish I thought were too far gone with the cleaner shrimp, even with the delicate, copper-unfriendly species. If you have animals that will eat cleaner shrimp, usually they will shred the fins on a Heniochus anyway. I have been keeping Heniochus on and off for 6 years now, including a school of 5 I used to have. They either die in the first week of purchase or they survive and grow at a very fast pace. Remember, Butterflyfish are not exactly the hardiest fish for an aquarium. Do not blame the forum. Blame yourself for buying a sick fish.