Tosfos says that we learn this prohibition concerning Gid Hanashe from Yaakov Avinu, who was alive when the Malach pushed in his thigh. Even those who forbid the Gid Hanashe or Chelev of a Ben Pekuah, agree that it applies only to the part that it was born with. When a Ben Pekuah is born, it is considered as if it's slaughtered and grown from a dead animal. The part that grows afterwards is permitted, since the Torah only prohibits Chelev or Gid from a live animal.
Therefore, we cannot be MiChayiv someone who ate the Chelev or Gid from a Ben Pekuah unless we can discern that he ate a Kazayis of what grew before the mother was Shechted.
We usually Poskin that if someone ate a complete Gid they would be Chayiv even if it was not a Kazayis, since it has the halacha of a Berya (a complete entity.) In the case of a Ben Pekuah, since the whole Gid is not Ahsur to him, you cannot MiChayiv him even if he ate the complete Gid.
We also cannot say that the permitted parts are Butal in the forbidden parts, nor that the whole Gid is prohibited. We can only say that Issur is Batul in what is Heter, but not that Heter is Butal in what is Issur.
