The first thing that I would do is to contact Thompson Center Customer Service. There is a phone number and a contact link on their customer service page:
http://www.tcarms.com/customerService/
I would ask them to help you identify the exact model. You may even need to obtain the model or serial number so that they can track down more info. about it for you. Ask them for some background info. like when it was made, how many were made, if it was a commemorative or what was significant about it? Also if you can get them to give you the orignal list price you might be able to better determine the original selling price or the price that your friend paid for it. I don't think that there's anything wrong with asking your friend about seeing the original receipt, or trying to search the data bases of gun auction sites like Gunbroker and AuctionArms to see if you can find any completed auctions for the same model and what they have sold for. It might be busy work but I'm not sure how else you can determine its current value better than to compare the prices that other people are actually paying for it on the open market.
Has it ever been fired?
Commemoratives can lose collector's status if they've been used so I would think that even a special edition TC would lose value if it was fired.
There is also a Thompson Center Custom Shop named Fox Ridge Outfitters who may know something about the model if they had anything to do with building it, or just because they're a TC subsidiary.
Also, since Smith & Wesson is now TC's parent company, you can also try climbing the ladder to see if you can get any useful information from them.
And there may be a current "Reference Book of BP Gun Values" much like Blue Book, but I'm not even sure which reference book to recommend for you to check.
There's an old saying, "If in doubt, don't whip it out!" I think that's referring to whipping out cold cash!
That TC gun may be very valuable but I doubt that the stock is green because it's made from money!
Good luck and let us know what you end up learning about it.
I wonder if your friend will let you take some pictures of it which you could then post here for us to see?