So sorry to hear this! She must be flipping!

I don't have any advice, just my personal experience with ON.

When I had ON, it was right eye, everything was gray, sort of like fog or the snow from an old black and white TV after the signal went off for the night. I could see a very small portion ( I'd look at someone and could only see their face, no hair, no body...) but even that was foggy. I had no pain at all and that seemed to surprise the Opth.

Did they do anything for it, steroids or anything? It seems odd that nerve damage would come on suddenly like that with no antecedent.

The optic nerve can recover from damage. I am living proof of that.

I hope she finds out what is going on. Please let us know and hang in there! You are a good SIL.