Powerwise, I see little wrong with making it a hex. That said, if it were my shoes, I would make him take that dip. The situation presented smacks of cherry-picking and hoping to avoid consequence. Make him take that dip, dint his spell casting progression and accumulate a curse and see if he still wants it. If so, then I don't see an issue in making it scale in level offhand.
Why the dip in Oracle rather than full-out? Necromancy is an area where divine can hold its own against arcane. If he wants to raise dead, then it is just as capable. Far be it for me to tell a player what to play, but it might be a good idea to find out exactly what about the witch he wants to play, rather than what he wants to poach from oracle.
A main reason I say this is that I know many players that despise the Oracle's curses, and if it were me, I would still stick the player with one.
To get back on topic; I doubt it will break the game, but I always like to see if I can find another answer first.