Beef in my stew isn't tender
There are two possible reasons why the meat might be tough.


The right meat, just cooked not long enough
If your son used "stewing meat", that is, rich in collagen layers and marbeled, you are on the right track. Simply cooking longer until the collagen breaks down will give you the desired results. And yes, a pressure cooker works perfectly for this. The higher pressure allows the liquid in the pot to boil at a higher temperature, thus reducing the cooking time compared to a regular pot. (A crock pot / slow cooker does the same in much longer time.)
The wrong meat, i.e. a lean cut
That is the material for a quick sear, often served medium or rare. Longer cooking results in the proteins in the meat toughening up (not unlike when you cook an egg), giving you a "dry" piece of meat. Unfortunately, longer cooking won't really help, you are missing the collagen layers that - when soft - cause the meat to fall apart into small juicy strands.


Conclusion:
I don't know what's in your stew. For stewing meat, by all means keep cooking. For other types, note that the consistency might go from "not easy to eat" to "almost uneatable".