My understanding is that if the bee is to be harmed by the DE it has to dust bath in it, or try and collect it like pollen, such as if it was on a flower. No one bee could take enough back to the hive to kill it off. Now Sevin is not like that at all, it will kill for a very long time and dusting near an entrance to an unwanted hive will kill the colony as small amounts get tracked in by the workers so if your chickens get mites or lice that is NOT the product for you to use. DE would be much safer, but I'll bet the best recomendation would be Rotenone. That way if the bees get into the treatment they will benefit, it's used on bees for their parasites, and it's safe, effective and organic approved for chickens for mites and lice. Very short acting, less than a day so you have to retreat every few days for up to 10 days to get all the new bugs that hatch out from eggs but works great.
I agree that some DE in your coop would not hurt, any bee that goes in there is going to get eaten anyway. I think your chickens will pick off more of your friends bees than any amount of DE would.