If its just showing up on one of the plants exclusively.. i would be tempted to cut her down.
However, I would try some fungicides in spray form.. as gmslave noted the milk option works.
Other alternatives I have used for mildew are Green Cleaner and Dr Zymes - I prefer the former. These are natural fungicide and pesticides you can use during flower.
See how that does you after a couple days and take it from there!