Quite clearly this is yet another case of Managerial 'Your problems are yours, and mine are yours'. If this software is in such an appalling state then it is a result of Managerial sins, either of commission (just fix it OK), or omission (I hired that man because he was the cheapest and he said he could do the job, it isn't my fault he made a mess, what was I supposed to do, check his work?). My advice, stand your ground, it is basically his problem, not yours. Do not take ownership of problems that are not yours. Sleep easy at night.