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My husband went to pick the comfrey to make some feed, but it has mildew. Can we still use it?

Yes I think it would be ok as I don't think mildew lives in water and after soaking for a few weeks it wouldn't be a problem. I've composted some with mildew before with no problems noted, I think any spores were killed by the heating up of the heap. I'd perhaps avoid using it on courgette and pumpkins though to be on the safe side!
The comfrey I had with mildew was growing in pots not the main comfrey spot and watering was a bit erratic, they were perhaps kept a bit too dry which I think led to the out break of mildew. Perhaps your plants are a bit on the dry side?