Nature does have a way. Parthenogenesis in egg laying reptiles in the absence of a male of the species. If this cross you speak of did happen, there are complicating factors. Such as, are you talking about a male ribbon and a female natrix, or visa/versa? Think about it. One of the females retains eggs without a leathery shell, the other lays them and they do have a leathery shell. The question is, would these differences allow the embryos to develop properly at all?

I suppose it's possible for a ribbon hybrid to develop inside an egg laid by a female natrix, but what about the other way around?