I get you're the messenger. But it doesn't matter how one off it is , the person(s) it was made for maybe think it's worth £3000 but that's not it's real world worth, esp for a used bike.
on top of that, if it is a one off, means it was designed, geometry wise for someone in particular, that makes it less desirable for resale. and thus doesn't hold it's original price. It's a bike, not art.
For the money your friend is hoping for, eBay/gumtree/FB are probably the places he wants.