This isn't a theory. In the final episode of Friends, Chandler says that the apartment was rent controlled. She sublet it from her grandmother, which was mentioned several times throughout the series. In the final episode, it was revealed that Ross lived there for a short bit with grandmother during college when he tried to make it as a dancer, which would've been the late 80s. It is highly likely her grandmother would've been living there in the 70s as you suggested