well you would have to sell lower (430) and hopefully buy lower. Do the math and decide if it all makes sense.
Selling now at 430. If sold at 430, then you take 120 loss per btc. If you think it will go to 310 (which would be a break even point....if my math is correct) then consider doing so (remembering it was 339 ish a few weeks ago....might or might not break that)
What kind of weird logic is that? If he will be able to buy at the lower price that he sold he would always make profit (minus trading costs etc). Why does it have to go to $310 to make sense for him to sell at $430?
Btw, by the same logic people who buy at $930, could only sell at $430 if they expected bitcoin to fall to MINUS $70

EDIT: My point is, whether he should sell and buy back or not has *nothing* to do with his average buy-in price. It is about predicting future price of BTC and has nothing to do with his transaction history

He said he bought at 550. So now its 430. If he sold now, he takes a 120 dive. In order to make up that 120, it needs to drop to 310 for him to buy back in.