That is why you limit the use of the Aprentice.
It would be a good combo for some of the tougher quests when you only have 2 or 3 players.
Also the low body points and slower movement should provide some game balance.
Also to recieve benifit to learn a spell the caster must obviously must be in LOS to the Half Elf, so chances are the HAlf Elf would not actually be able to learn every spell cast in a game due to LOS issues.
To add in some more balance you could require the Half Elf to roll anything but a black shield on a dice to learn a spell.
Personally as the GM, I make it my personal lifes quest to murder off the Wizard first chance I get, so if the Wizard gets smoked, there will be no spells for the aprentice except for the few the Elf casts.
(yes I consider it my holy mission to completely wipe every party of adventurers I can. Makes me glow.)
So it is said that if you know your enemies and know yourself, you can win a hundred battles without a single loss.
If you only know yourself, but not your opponent, you may win or may lose.
If you know neither yourself nor your enemy, you will always endanger yourself.
~Sun Tsu The art of War~