I don't think there is anything you can do to make them food safe as they are most likely dyed and probably treated some way to clean them and rid them of parasites (as I assume they are pretty nasty when coming right off the bird, lol).
If it were me, I would proooobably not worry about the food safety part if they are only touching a fondant covered cake as people, from what I hear, don't usually eat the fondant. Is she wanting some of them sticking out of the cake? If so, I would get a hollow plastic or cardboard dowel and make a sort of sleeve for them. This way the...can't think of the word for the part of the feather that actually attaches to the body...lets call it the stem, that way the stem never touches the cake. Personally that would gross me out, food safe or not. Having them in a sleeve of sorts seems to make sense, to me anyway.
Hope that helps 
How about creating a dummy tier and sticking them in that? That way they are not touching an actual cake anyone will eat? Just a though
Thats a good idea...feathers in cakes freak me out. It just strikes me as a gross thing to do. I wouldn't have the tail of some other animal sticking out of a cake, or a tuft of my own hair - no matter how well it'd been washed. Same kind of thing when you think about it! But yeah, I get that people some people want feathers on their cakes and don't think there's anything gross about it. So putting them onto a dummy tier would be a good compromise, I could almost (almost!) cope with that.
I don't see anything wrong with using feathers on a cake. I have had a few brides to use them and they are not a problem. When we get the feathers in we steam them just for safety (my mother taught me this for cleaning mattresses and couch cushions, that steam can kill bacteria and freshen things. I'm not 100% on the statistics of this method, I just do it as its better than nothing.). After steaming the feathers (they will fluff back out) we take white floral tape and start taping the spine from the bottom upwards gathering any loose strands. Occasionally we will snip some of the strands to shape the feather. After taping the bottom of the spine and shaping we insert either bubble tea straws, regular straws, or the Wilton flower picks into the cake, depending on the design and pass the feathers through those. The straw should be a little longer than the end of the feather as to not pass the feather beyond it and into the cake.
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