You're stuck with it. Attempting to unfuse it will leave you with a bad egg. You can't release it. Enjoy your illegal figurative paperweight. Tell your "friend" to use his AR to delete it. If you downloaded it with a fakeGTS service then you're screwed.
FYI this is exactly the reason Pokécheck does not allow users to download fused Kyurem via GTS. --- http://i.imgur.com/qCo47.png