I don't think it would have been Younger, because she still might be appearing in future episodes. (She plays one of Sutton Foster's friends, and she pops up once a season or so.) I would hope she wouldn't trash an ongoing employer. Besides, Younger is pretty fun; it's not great art, but I wouldn't call it "semi-terrible."
Besides Younger, I know her from having had a decent-sized role in Next Fall. She's also appeared in a bunch of other theater in NYC over the years.