Assuming he purchased it from an authorized online retailer they likely have a record of his transaction.
I am assuming you have been messaging me on Facebook as well ? So I can simply reply in one place.
The warranty is very clear and simple -- by technicality only the original owner has warranty on woofers but I am currently extending you an offer that if your friend can simply get his documents together that it will still be taken care of for you as the 2nd owner. We will need some sort of documentation, though, and I do not think that is too much to ask for warranty service.
Again... as I just mentioned on FB if you can even get his name and the dealer he got it from it can likely get solved quickly.