There is no need to add “on” in front of tomorow, yesterday, or today. Even if it is a colloquialism in southern Virginia, it is not correct. I think the larger problem is how you will approach this co-worker and tell her she is wrong without alienating her. I might suggest you blame me or note that national English is larger than local English. So if she wants to run for office someday, she might want to be more national in her language. Even if she doesn’t seek public office, knowing correct grammar acknowledges her as a critical thinker.