It would kill Thomas, his Demon is the only thing keeping him alive.
Demonreach can parole a prisoner, Harry could release Ethnui but bound to his orders, so he actually has an island full of superweapons not just an armoury. He can probably separate a Mantle from the holder, and release the holder without the Mantle, but this would result in someone like Molly being left incapable of functioning as a human being. Same dilemma as Susan, same solution. An unravelling of the Mantle.
Thomas needs to be released and fed immediately with the richest most inexhaustible life force available, a normal human would not be enough, it would kill them and would be a deal breaker. The Winter Lady, an immortal might do it, and would probably survive the ordeal. Mab would be even better and would definitely survive. As Mab one said about Thomas becoming the Winter Knight “Human enough” so I suspect it goes both ways, Mab and Molly both originally human are probably “Human enough” for Thomas to feed on them. Mab would probably approve of having the human sucked out of Molly.
Harry has The Shroud, so he could use that to sustain Thomas during the feeding.
