The father who is on the hook for the loan may need a little more information as to the benefits he will see on his credit if the short sale is approved. Actually there are pros and cons to the short sale. The con can be an unplanned tax liability.
Both borrowers need the whole picture so they can make an informed decision. Often when someone is being difficult it is because they don't understand a lot of what is going on so they naturally don't want to make a decision.
I would bullet list the benefits and the downside of doing a short sale and the same with the foreclosure. I would then explain these to the father. He is then either on board with it at that point or he's not.