I would assume that the caster of the spell that is being eaten takes damage, as it would be redundant for the caster of spell eater to take damage. However, that would mean that Spell Eater is literally just a better version of Dispel. I would run this by the Rules Thread (top of this page), and ask Joe himself. Because, here's the thing, both Dispel and Spell Eater seem to do the exact same thing, and have the same target number, except Spell Eater also does damage. I always played under the assumption that Spell Eater does damage to the target spells initial caster, but I think it would be better, if the caster gets a bonus to his/her next roll on any spell next turn, because they successfully ate the spell, consuming its energy, but I also think Spell Eater should either have a higher target number, or a change in range. Either way, I say run it by Joe, if you don't want to I can and then report to this thread what he says.
Thank you,
Like Clockwork