It's a very strange case indeed. I have two solutions:
1. Try to get a calcium reading, ideally it should be 400-420ppm. Since favia have such large skeletons they tend to suck back the calc. Perhaps that's one cause for it's issue.
2. Favia don't need to be fed, but it sure helps with growth and in this situation, recuperation. They have skinny, transparent feeding tentacles that will hopefully emerge when they sense something meaty in the water around them. Try mysis or brine shrimp, just spot feed with an eye-dropper or even a turkey baster. The shrimp will stick to the polyps and hopefully the coral will absorb them.
Anyway, good luck. I wish you the best