Yes they will be able to leave the airport and head into the city and this is indeed what they should do. If they have checked bags they should ensure they put them on the connection belt before leaving the airport though.

Access to the city is pretty quick, easy and inexpensive on the train (around $5 one way IIRC). Once in the city there is loads to do. If they are interested in American history then Philly is great and if they are into Rocky films they can climb the famous Rocky steps (having never seen a rocky film I wouldn't know anything about this). It all depends on what they are interested in but they won't go bored.