A really good question:
Suppose you think (like most people) that an action can (a) have unintended bad consequences but (b) not be morally wrong. If there are such actions and you perform one, I'd think that you have a prima facie duty to repair its consequences that is not a "mere duty of beneficence". I'm tempted to call it a "reparative duty", but clearly it wouldn't be if such duties only arise in response to past wrongs. That's just a terminological problem, though.
So that's the kind of duty Plum has to help Cook, and she doesn't have that kind of duty to help Mustard. That's why (a) is right. What's wrong with saying something like this?
Here's my answer. It's sketchy and I'm short on sleep, but I think the question really helps focus the discussion. Here goes. First, we have to figure out what 'wrong' amounts to. Let's say that we reject the closure view of wrongdoing and say that an action can be wrong without being all things considered wrong. On this usage, all that 'wrong' amounts to is this: there was a pro tanto moral reason not to do it.
Second, we have to figure out what your proposal would amount to if 'wrong' is understood in this way. It seems that the proposal would be that there can be reparative* duties in the absence of wrongdoing (I'm using the '*' to indicate that these are more stringent duties that apply only to some in the absence of a previous wrong on the relevant agent's part). That is to say, you can have a reparative* duty to rectify some bad state of affairs that others aren't under even though there was never prior to this time a pro tanto reason to refrain from bringing about this bad state of affairs. But, this is what's odd. If there were two people who could help, one who caused the bad state of affairs and one who did not, I take it that you'd want to say that the causally responsible agent is under some reparative* duty whereas the other is under some mere duty of beneficence. But, if there was never a reason for the first to refrain from bringing about the bad state of affairs, why would this be?
Now, much of this is sketchy and you might have a different sense of 'wrong' in mind. Obviously, the more you read into 'wrong', the more plausible your suggestion is. The less you read into 'wrong', the harder it is to work out the details of the view that I think you're proposing. But, it's a good question/suggestion.
Any thoughts would be much appreciated.