If you are a guardian of your father, then, yes, you do need to file some type of response for him.  If you do not have some document that grants you the authority to do so, then you need to get something asap so that you can either file a response pro se or have an attorney file one for you.  If a default judgment gets entered against your dad, then you need to get an attorney involved to file a motion for reconsideration or motion for new trial.  Considering that the plaintiff served notice on a person who was not legally capable of responding, you would have a basis for getting the judgment set aside.  You may be tempted to throw up your hands, but getting the judgment set aside is important for your father's finances and his estate finances.  With a judgment, the creditor could potentially attach what few resources he has left to support himself with.