I can't imagine the events that proceeded a garment with an 8 carat diamond in the pocket being donated, but the one thing that is for sure is that it wasn't done on purpose. I'd feel like I needed to make an effort to reunite it with the real owner. If the owner isn't found, maybe your pal would get to keep it. Even if the owner was found, he probably gets a hefty reward. Plus, from a practical standpoint, what is he going to do with an 8 carat diamond? He can't exactly roll into Kay Jewelers to see how much they would give him for it.
Reminds me of the time I was at the jewelers getting my wife's ring sized, and some guy rolls in with a giant hunk of uncut gemstone (forget what it was) he said he found in an attic and he had estimated at over a million. Jewelers were kind of perplexed and said they really didn't deal in that stuff and he needed to go to an auction house. No idea if it was real or not but it seemed sketchy as hell.
Feel like any legit jeweler is going to want proof of ownership so they don't get burned.