My opinion from experience is that, no they can not get out of the contract. But, there could be some exceptions. Why is it delayed, Mortgage problem? There may be a mortgage contingency in the contract that states if they can't get a mortgage, then they can be allowed to exit, but there can be deposit issues for the buyer, as in the buyer can possibly lose their deposit.
Time of the essence can be served to the buyer, bascially the sellers attonrey telling them if that they do NOT close.. there wil be ramifiations.
There are a whole host of things that can happen. Speak with a real estate attorney or your title company to see what is really going on.
Let us know some more details.