OP, first, I would be upset too, on behalf of your deceased friend and son.
Second, 13:23 is correct, but doesn't understand that the delivery matters. 13:23 please google Marshall McLuhan.
So I'll just say this: The money is the husband's and not owed to the child; and the child should be expecting nothing. The husband did make a promise to your friend, but both were at the time unknowing that this promise would then get in the way of his happiness with his new mate. Your friend would not want that. If there is a heaven, then she can see what he is trying to do, that he is trying to build another family for himself and their child.
His actions may end up being misguided; he can then correct that; documents can be changed. But it's a way to give him, and his son, the best chance at this new arrangement.
So I think, OP, that this is a situation where reasonable people can make different choices, and although it's not a choice that you (or I ) would make, it's understandable.
Finally, depending on how well you know the DH, you could go to him and confess the elephant on the table, leading with that it's none of your business, but you are coming to him to help you reconcile how you feel --happy for him, worried about his son, trying to deal with knowing what the arrangement was at your friend's death....see what he says