the base is about 8 inches across with fronds up to 18 inches or so

Looks like Platycerium bifurcatum, they're quite easy to grow if you keep up the humidity. Whether there's any damage to the plant just depends on how gently (or otherwise) it was "ripped off". Mostly these plants will survive given a bit of TLC.

Nest leaves grow out green but normally turn brown and are in turn eventually grown over by new nest leaves.

Are you going to mount it in a tree, or on a board?

Well, I don't have a tree, so a board. I have it sitting 'tree-side' down on some damp coco chips, while I figure out what to do with it. The store was out of spaghnum moss when I stopped last night.... ahhh hawaii. :P I will have to hunt around this weekend and mount it then. I also have to figure out if my home or lanai is humid enough, my office is in a rather rainy valley, but I live in a much sunnier and windier area.

You could do it without the sphagnum. Until it establishes itself just keep it out of the sun and wind. I don't use sphagnum and it would be a lot hotter with lower humidity for most of the year. So for me even though they struggle along a bit they're still okay but should do a lot better for you.