You are correct, only God knows the day and the hour of the Rapture!
That's easy. If the cannibal later gets saved and is still living at the time of the Rapture, his body will be transformed into a Heavenly body, like all the rest of the ruptured and resurrected believers.
The meat that his body absorbed became his own so nothing happens there.
The part that his body did not absorb, the waste, he crapped on the dust of the ground somewhere and it became part of the earth.
God formed Adam from the dust of the earth. So God will have no problems reconstructing the bodies of those believers the cannibal ate from the dust of the earth, resurrecting them and finally transforming their earthly bodies into Heavenly bodies.
If the cannibal is dead at the time of the Rapture, then all the easier.
Puzzle solved!
I don't know how it is easier. The bodies in the grave will rise first, right? So all those earthly bodies laying in the dirt will rise up and meet Jesus in the air. If the cannibal is alive when the rapture takes place, then his body will have to go up to heaven, because his body is someone else's body whom the cannibal has eaten. So the cannibal will be without a body, correct? It is only when we get up into heaven that we will receive our new glorified body, right? The cannibal will have to have something to go up and meet Jesus.
If the cannibal is dead, then I don't see how the puzzle is any easier. It seems we get the same problem, "who will get what body?" If you want to say that God will create a new body for the cannibal, while the cannibal's flesh is torn off to give back to the previous alive saved people, then you have to explain that.
What if the cannibal is not saved, but those he had eaten were saved, and the cannibal is alive at the time of the rapture? The cannibal's body will have to stay here on earth, because only the saved are going to meet Jesus, but the dead in Christ will rise first, so the cannibal's body (the meat that he has absorbed throughout a lifetime of christian eating) will have to go up to heaven and be reformed into the body of whoever he had eaten, correct? So how can the cannibal stay here on earth without a body? Again if you want to say he will get a new body, you have to explain why, because that is not in the bible.
I do want to say that I may be talking out of my ass here. I haven't given it much thought, just however long it took me to type out this post. So theres a good chance I'm missing something easy.