Am I reading this creature's stats correctly? Even +1 or better magic edged weapons don't harm it? Only blunt?
Also, if it's immune to elcetrical damage and has enveloped a victim, would the victim take damage from someone casting Lightning Bolt at the creature?
[...]slashing or piercing weapon attacks [...] do no harm to the aballin itself. only bludgeoning weapons can harm the creature (and ONLY if they overcome its damage reduction
As for the lectricity damage, i'd say yes. a ligthning bolt doesnt bounce off the aballin, it goes tyhrough it, electrocuting the entrapped victim. I don't want to go into a pseudo-scientific explanation (because they often don't really apply) but here, an aballin is made of water, which is conductive. If it were made of an isolant "material" you could rule that the entrapped victim suffers no damage as well.
In fact, i believe that a fireball wouldn't harm the entrapped victim, but a lightning bolt would... As for a cone of cold...hmm... THAT would be the real question, since the aballin isnt even slowed by cold energy attacks...