If a hero is grappled on a square next to a naga, can another pass through that same square to get somewhere else? Would they be stuck in the same square, or would the move just not be allowed?

The gathered list of answered questions thread has this to say:

Grapple
A)Is a hero with Grapple really intended to keep 8 creatures in one place?
B)Can a fiendly figure move through a space occupied by another friendly figure that is currently being grappled?

A) Yes. B) Yes. Grapple can only hold one figure per space, however.

I can't find anything that says this exactly, but it was my recollection that you simply cannot grapple a figure while it is in any position where it would be illegal for it to end its movement.

Antistone said:

Completely the right answer, just to have a second on the ruling.