An acquaintance of mine has had this piece for years and has yet to figure out what it is. Any ideas?

Very difficult to ID from those pics alone. My first inclination is Wyoming Jade, but again impossible to tell without further data.
Factors that will help you narrow it down include general location of find (investigate the geology of that area), simple specific gravity test, and MOHS hardness test.
Very cool piece!

Here are a few more pictures. I'll ask her about location.

hey Nate,

I suggest you look at the possibility it might be Slag, "Railroad Slag". When I first look at the pictures, that's what I see. It comes in many different colors and green is a very common one.

I have several pieces laying around that look a lot like that but can't seem to find them at the moment. Here are a couple of pics from the internet. Have a good one, Woody.

That was my guess too! Haha

It could very well be slag, especially if RR tracks are anywhere near the find. Looking closer at the pics I see what appears to be bubbling, etc., typical of slag.
Is the piece heavy or light for its size? I would encourage you to try this: Simple Specific Gravity Test

Looks like slag Nate. Specific gravity test or hardness may not be helpful enough to be conclusive.