Either we have access to the moral principles to judge benevolence (moral goodness) or we don't. If we do, then we can judge God's actions based on whether they are in accordance with the principles of morality. You can say that God isn't subject to the principles of morality, or that she is subject to some moral principles, but those are beyond our understanding. God has more moral depth than we can know. But what you have done then is reduced the claim "God is good" down to gibberish. What does it mean to say that God follows morality that is in principle unknowable to us when its clear she violates what is known to us? Saying God does the right thing, but we don't know what the right thing is no different practically than saying whatever God does is good because God does it. If we are going to say "God is benevolent" that statement is going to have to generate some expectations for us if it is to be meaningful at all.
So if you want to say a god exists with certain properties, God's ways must be knowable to us, at least to some extent. We must be able to say, for example, that it's generally wrong to allow a person who's dying anyway to suffer horribly if one can prevent it with no risk or effort, and that therefore if God does so routinely, in almost every case where she could have prevented it, she has almost certainly acted wrongly in some of those cases, unless we have good reason to think not preventing it served some greater good. If not, what exactly do we know about morality then? ... Similarly, if it seems transparently clear, according to our human notions of justice, that it is unjust to punish someone infinitely for a finite offense, especially under circumstances where it cannot serve to rehabilitate the offender, where his punishment cannot possibly serve as an example or deterrent to others, and where he cannot possibly have made an informed choice, we must be able to say that it really is unjust, or else admit that we have no idea what justice means when speaking about God. [EA: This was orginally addressed to an Evangelical]
You have to be able to show that the way God operates is at least plausibly the way we might expect an omnipotent, omniscient, omnibenevolent being to operate. You can't deflect objections of this sort by simply observing we don't know how God would do things, a.k.a. the ways of God are mysterious. If God-theory cannot inform our expectations of how reality is supposed to look like, then you've reduced God to a shot in the dark.
Chances are Bob wants to say something like "God is good." But what he means by "good" isn't what we ordinarily mean by it. His version of "good" doesn't contain meaningful content. Instead is a just a trite tautology that reduces into "God does what God does" as he's chosen to dismiss moral criticism of his proposed ideas about God by stating that God's just purposes are beyond our capacity to grasp. And that's gibberish as far as trying to figure out what God will do or what it means to be moral as far as God is concerned. And if he were to base his views on what is moral on what God does, that' would be completely arbitrary, as he'd have no moral basis for picking God over any other subject like myself. A wiser person, or at least one who has studied basic metaethics and learned a little bit, probably should just admit that such things damage the believer's case, but not irreparably so given all this other reason to believe in God's goodness that exist. (Of course, it doesn't, but let's play pretend for a moment.)
