A friend of mine frequently uses the expression "Dorothy Mix", usually referring to the number six. Does anyone know the origin? Is it perhaps Cockney rhyming slang? Was she a real person?

In Australian rhyming slang ,the expression "Dorothy Dix" refers to a hit for six in cricket. Australian rhyming slang has it's origins in Cockney rhyming slang . Most often these types of rhyming slang use substitute words, usually two, as a coded alternative for another word. The final word of the substitute phrase rhymes with the word it replaces ie."Dorothy Dix"= "Six". (Dorthy Dix was was the pseudonym of U.S. journalist Elizabeth Meriwether Gilmer.Her advice on marriage was syndicated in newspapers around the world making her a popular and recognized figure on her travels abroad.) I'm unaware of any prominent person named "Dorthy Mix" but by the way your friend uses this expression it sounds as though it could be a rhyming slang variation of "Dorthy Dix = Six" or perhaps it may be an original Cockney rhyming slang expression that the Australian rhyming slang phrase " Dorthy Dix " derived from.

"Tom Mix" was also used as a cricket term for a six, so perhaps your colleague has blended Dorothy and Tom
https://en.wikipedia.org/wiki/Tom_Mix