The difference is not in how common the occurrence is but whether the aid is extraordinary or ordinary.
If instead of being sewn up with a violinist, the person in the thought experiment woke up with an embryo implanted in their uterus, then separating themselves from the embryo would be removing ordinary aid and the embryo would die from being deprived of basic nutrition.
This latter case is what happens in e.g. date rape, which is, I think, the whole purpose of the "violinist" thought experiment to begin with.
If instead of being sewn up with a violinist, the person in the thought experiment woke up with an embryo implanted in their uterus, then separating themselves from the embryo would be removing ordinary aid and the embryo would die from being deprived of basic nutrition.
This latter case is what happens in e.g. date rape, which is, I think, the whole purpose of the "violinist" thought experiment to begin with.