Let's break it down, this guy left mugen. He cut all contact with everybody, and he never said you couldn't do it (neither did he say you could). First off all, since he said he's leaving mugen, and he put up content for download, what does he expect everybody to do? Leave it alone? If he were really worried about it, i'm sure he would've taken it down. Let us observe the old axiom: 'Silence shows approval.' If you in fact asked him use the sprites and he said nothing, then his silence showed that he was indifferent or he approved. He has no argument to contradict that, he has no right to be angry if you do use them (if he does come back and gets angry). So, really you can use them. Like a man who dies without a will, all his possessions are given to the state (at least it was in antiquity). If you are worried, the least you could do is credit the use of his sprites.