@breizh
@Nakewameke
since it's a named disease, there are likely previous victims and research about the disease so I'm assuming he has looked up the previous cases, including what erasing the 'e' in 'emeth' actually does. so
@aberdeus 's explanation is most likely correct in the case of this disease.
also, it's possible that all her monster forms can be 'killed', and she'll return to be human after her monster form is killed.
I'm betting she'll transform into sphinx once, and he turns her back by answering her riddle correctly.