You're browsing the GameFAQs Message Boards as a guest. Sign Up for free (or Log In if you already have an account) to be able to post messages, change how messages are displayed, and view media in posts.
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.