As you've described the case, there's something the inventor could do that would save lives. There's also a dispute about how to analyze the notion of a cause. Some would say (your friend apparently is in this camp) that absences—in the case, not doing something—can't be causes. Others disagree and provide accounts that allow absences to be causal. This is an abstract and complicated issue, but how much difference will it make to how we judge the inventor?
Suppose I'm in a war zone and happen to know that there's an IED in a certain spot. I see someone running on a path that will take him over the IED and almost certainly leave him dead. Let's assume I even know who it is and know that in all relevant respects, he's an innocent. As it happens, I'm behind a barrier, but I could easily warn him. I don't. He runs over the IED and dies in the blast.
Is there something I could have done that would have saved him? We've already said yes. Would it have come at any significant cost? We can stipulate for purposes of this example that the answer is no. Should I have warned him? I'd say yes; I'd hope your friend would too. Was my silence a cause (note: I didn't say the cause) of his death? Why, exactly, does it matter beyond what we've already said?
Change the story a bit. Before he gets to the IED, the running man yells to me: "Is it safe ahead?" I lie and say yes. Was my lie a cause of his death? Maybe there's less dispute about that in this example. In any case, there's likely to be more agreement that the lie is even worse than saying nothing. But unless using the word "cause" is just a way of talking about culpability, it's not clear how much difference worrying about the fine points of causation makes.
We can imagine other variations. We might think that even though the lie would make me even more culpable, I'd still be less culpable than if I'd shot him. There are interesting and tricky questions here about responsibility. And for those whose tastes run to analytic metaphysics, there are interesting and tricky background questions about the best way to analyze causation. But my hunch is that in the case of the inventor and in the case of the IED, trying to sort out the metaphysics of causation is likely to be less useful than starting with what everyone can agree on and worrying about the question of responsibility in the morally weighted sense. In the case you and your friend are arguing about, everyone can agree that it's in the inventor's power to save lives. I'd hope we'd also agree that it's wrong of him not to. We can also agree that there are even worse things he could do—like helping to spread the disease among the very people he's chosen to keep the cure away from. But once we have all of these sorts of details on the table, it's not clear what's added by taking sides on whether or not to use the word "cause."