I've done the SWF (Salt-Water-Flush) on occasion while not fasting, if I feel the need. I still do it only first thing in the morning, on an empty stomach, as directed in Burrough's Master-Cleanse book.
If people use enemas and laxatives /lax teas when not fasting, I don't see how the SWF (Salt-Water-Flush) is any different. Burroughs calls it "an oral enema".
It has worked just fine for me, with no ill effects.
I don't know that it would help someone who was already sick, though.