1. The validity of the argument isn't affected by whether one is a hypocrite. If Apple is indeed behaving unethically, it's still behaving unethically even if I own an iPhone. If Apple isn't behaving unethically, then you should be able to argue that on grounds other than "you bought an iPhone." The poster in this case made specific, refutable claims - that Apple dodges taxes and can't pay decent wages to their Chinese factory workers. A legitimate counterargument is that they don't in fact dodge taxes or do in fact pay decent wages (or perhaps that their current behavior is okay), not an attack on the person making the argument.
2. Apple products are not a necessity, but in the current era, a smartphone is very close to a necessity (yes, there are people who steadfastly avoid them, and there are also people who live in the forest, that's not what "necessity" means). And there are relatively few options for good smartphones, none of which come from morally blameless companies. (Personally, I find security valuable, and Apple has the best track record of producing phones with long-term security support, which means you can buy fewer Apple products than anything else.) It might be hypocritical to buy a product from company X while steadfastly believing that company X is behaving unethically, but it's hardly a sign that you don't in fact believe that company X behaves unethically. You might simply have weight the alternatives and decided that buying from company X is the best of several bad options.