While nothing undoes the effects, one could increase their intelligence through other means. For example, if the person found a Headband of Intellect, that would put them back up at a perfectly respectable 19 Int, and is probably the quickest way to have a player rapidly regain their Int score.
3) The greater restoration spell could likely do it. The card is an instantaneous magical effect that permanently reduces your intelligence. However, it isn't an ongoing magical effect, so if a different effect comes along that would change the intelligence then it would do so. "Permanent" could be interpreted as "Fixed and unchangeable" or it could be interpreted as "Set to this new value until something else changes it".
I view it as “set to a new value unless something changes it” much like an Ability Score Improvement at level 4 or 8. So, a wish could do it in that Wish might be able to change your ability scores.
It’s a legendary magical item and explicitly a gamble where things can go very wrong.
Many of the cards mention how to counter or undo their effects, not Idiot.
By what means would you allow Idiot's effects to be undone?
I wouldn't.
While nothing undoes the effects, one could increase their intelligence through other means. For example, if the person found a Headband of Intellect, that would put them back up at a perfectly respectable 19 Int, and is probably the quickest way to have a player rapidly regain their Int score.
The fates card. Maybe a wish.
There are a few ways ...
1) The Fates card from the deck could undo it.
2) A DM might allow a Wish to undo it.
3) The greater restoration spell could likely do it. The card is an instantaneous magical effect that permanently reduces your intelligence. However, it isn't an ongoing magical effect, so if a different effect comes along that would change the intelligence then it would do so. "Permanent" could be interpreted as "Fixed and unchangeable" or it could be interpreted as "Set to this new value until something else changes it".
4) Headband of Intellect
...
I view it as “set to a new value unless something changes it” much like an Ability Score Improvement at level 4 or 8. So, a wish could do it in that Wish might be able to change your ability scores.
It’s a legendary magical item and explicitly a gamble where things can go very wrong.
Divine intervention.
But that could end up just being the location of the magic item or the location of the wish spell. Leaving it up to you to acquire it.