Apple may be Foxconn's biggest customer but this isn't just Foxconn and it isn't just China.
Put this in the perspective it really needs - what percentage of Asia's comparable manufacturing output do you think Apple accounts for?
And now consider that it's not just Asia, it's countries all over South and Central America and Eastern Europe.
A friend of mine used to be a town planner and part of his job was getting planning permission for new McDonald's sites. When you're pitching to the relevant authorities in the UK, you're under no obligation to reveal who your customer is, just the basic nature of the business (in this case fast food restaurant). But, he said, they would often hint (in a deniable manner obviously) that they were representing Burger King because McDonald's had such a bad reputation.
That reputation while largely deserved, distinguished them only from what we'd want them to be like, not from their competitors. Any fast food chain on that scale at a corporate level is largely indistinguishable from McDonald's - it's how they remain competitive.
But McDonalds' rep meant that they got the kicking so people would rather have a (morally identical) Burger King in their town. Cosmetically a win perhaps, in reality nothing got any better anywhere.
What I'm saying in a very roundabout way, if you boycott Apple, if all you do is move your custom to another organisation who are no better, you've made close to no difference at all. If this really bothers you, then you probably need to look further afield for your next smartphone or PC or tablet than the next big player.