With a tamed Fiend I believe you can just drop food on the ground and they should eat it like other animals. Additionally they should be able to access the backpacks of characters in the same squad so if you have food there they should eat it. I have some camping mod with raw meat storage and I've seen my bone fiend eat from there as well. They'll eat corpses but it's hard to tell if this actually fills their hunger or is just partial cleanup as they don't eat the limbs like bonedogs.

Lastly you should be able to equip them with a backpack and put food in there like a packbull/garru, though I've been unable to find any fiend backpacks for some reason, maybe conflicting mods idk. If anyone knows where to find a bag for the little fellows, I'd appreciate the advice.

^ finally found a fiend backpack at World's End, should be pretty reliable way to get it there since they have a ton of different animal packs from what I saw. Tamed animals will eat any food, thankfully no dietary restriction in this game