best case scenario: its poop you smell and the snake is still alive; worst case: your BF drives the rolling casket/stink-mobile as long as he can then sells it for scrap b/c NOTHING will get rid of that smell. Either way, i'd say your best bet is to remove and disassemble/de-upholster the seat it originally crawled under b/c it easily could have wedged itself into the foam and structural elements. Obviously you will need to completely replace the seat, but if that is actually the location of the tomb and/or toilet, the smell might just leave with it.