I noticed in a couple of episodes Rhoda coming home from work and having dinner at Mary's apartment. Was this a regular occurrence?

Yeah, pretty much. Rhoda's apartment was probably too small for her to do a big meal like Mary's. :lol:

It also gave Mary a chance to discuss her day and what was going on at the newsroom. Since Mary didn't have a spouse, Rhoda played that role in those scenes.

It appears they had a nice friendship.

I seem to remember it was. They were certainly in each other's apartments A LOT in general, for sure.