As the character regaining her sight would likely only happen due to GM fiat, I’m inclined to leave that up to the GM. However, I’d recommend it be treated roughly like becoming an ex-cleric, ex-paladin, or ex-druid; the character loses all Eye abilities except the spells per day and caster levels provided by the Eye levels (as those really represent advancement along the original spellcasting path). If the Eye was originally a cleric of Boccob and intentionally heals her blindness, I’d recommend losing the cleric abilities and all spells as well.

Also, what happens if the character regain his or her sight somehow? Are any or all of the prestige class abilities lost?

In short, there’s no game balance issue and no flavor issue, so I’m inclined to err on the side of player choice. A GM would have to keep a close eye on this class anyway if it was ever used for a PC.

Of course, the choice in NWN2 may be to simplify programming issues.

As for the spells per day, see the Arcane Trickster, Archmage, Loremaster, Mystic Theurge, Red Wizard, and Thaumaturgist prestige classes from the 3.5 DMG. These classes all provide the same spell progression as the Eye and also grant powers above and beyond the base magic-using classes, but they have specific requirements before they can be used. The Eye’s requirements are especially heinous.

I’ll add text clarifying the spellcasting progression. The DMG is about as vague on this, but the intent is clear: you can pick which class to add to at each level.

You may also want to clarify whether you must choose which spellcasting type and stick with it or whether or not you could switch between several (for instance a cleric/wizard would have to choose one and stick with it through the full Eye of Boccob level progression).

If only it was ever possible to use this ability…