They been flushed for 6 days now, grapey Walter #2...

There's nothing that you can do at this point. The burnt tips are a clear sign of over feeding. That only one plant is exhibiting them, only means that the others were a bit more resistant to the over feeding; if you look at the past weeks, there's a persistent dark green color on the foliage, characteristic of nitrogen toxicity. For growing in soil an EC/PPM meter is a very useful tool, but for coco and hydro it is a must. Two weeks flush is too much for coco, there's no buffer of nutrients in the media for the end of the flower period, 1 week is enough. although in this case just continue to feed water only so the plant consume the excess of nutes it has already accumulated in its foliage. Good luck mate