If W-2C does not arrive by April 15?

My wife is getting a corrected W2.  (They sent us one earlier, but it is incorrect and I have discussed with the company.)  We haven't received the correct "corrected" W2 yet.  I still need to do my state taxes...

If we don't receive the W-2C by April 15, what should I do?  File for an extension?  (Does the company that messed this up to begin with have any liability for this?)

Thanks.