Since he flat called the turn I think he can safely fold the river. His opponent should be worried that he called with the
unless he has it himself, which he probably does.
The turn really needs to be raised and it is completely untrue that you won't be called unless you're beaten. If he has the
he'll call, if he has a smaller flush he will call or raise, if he was slowplaying a set or two-pair on the flop he'll call.
I don't think you have to bet the flop. Taking a free card to draw to the flush is totally OK, you don't need to bet your draws every time, especially not in multiway pots.