Well the first step is knowing more, so that he can truly make an informed decision. I do agree that selling it right now is NOT a good idea. A big known issue like that is the ultimate in "sales prevention". Any offers will be very very low indeed. In PA, the seller needs to provide the buyer with a CO (certificate of occupancy), so I'm afraid that a major foundation issue may prohibit this from being a possibility.
He should get a few good masons to come out and take a look at the situation. Some of them will be full of BS and try and give him all sorts of high estimates etc. But he'll get a much better understanding of what's wrong, and various options to take care of things. If you get a good mason (and many of them are either out of work or not fully employed right now) - then you both might be surprised what might be possible with even only $5K - $10K. The materials really aren't all that pricey. Even the labor cost should be reasonable. But you need one person that leads the project and really knows their stuff, to supervise installing new footings, then very CAREFULLY jack in new supports between the new footings and the above structure. It CAN be done and it need not be overly expensive.
My next door neighbor happens to be an excellent and skilled mason - I'm only in Bristol, so just North of Philliy. He's pretty busy, but perhaps he might be able to come out and give his professional opinion as to what options your father really has, or if he's too busy to recommend someone for you.
If I saw a house for sale with a bad foundation I would not pay for the house period. I would offer about what the lot was worth maybe taking some off for the cost of house removal.
To replace the foundation you have to jack the house up., Then yes, spend lots of money. Often when the house is jacked up ( read almost always without a great company doing the job) the walls will shift and the drywall will break or crack. So if the house has many problems now it is almost certainly better to tear it down and start over.
Since the house has had nothing done since 1978 I wonder... does it have any insulation, is anything rotted, is the plumbing ready to break, is the wiring anywhere near up to code? Are the windows anything close to energy efficient? To sell it now for a good price would it need a whole new kitchen and bathroom? Those are the most expensive to change over.
It always costs more to fix up a real bad house than to build new. Labor is just that way.