The Clear Spindle Ioun Stone keeps the wearer sustained, but it doesn't explicitly say it prevents starvation/dehydration. Feast of Ashes/Cup of Dust apply the starvation/dehydration effect. I'm unsure how the spells react if the target is using the ioun stone.
My party's witch feels that the spells would still take effect as there is no 'sustained' effect and the stone doesn't explicitly say it prevents the starvation/dehydration effect. I'm unsure as the intent of the stone is obviously to stop those effects, but the interaction between the constant effect of the stone and the spells isn't clear to me.
Anyone know the official way to handle this?
The stone essentially emulates always being well fed and having drank enough water. That is to say, you will not become dehydrated due to not drinking and you will not starve due to not eating. That's all it does in my book.
If an effect causes the starvation effects by some other means, then I don't think the stone protects against it. The spell simply causes extreme hunger which causes the effects of starvation, and I don't think the ioun stone prevents that.
Of course, there are some differences between the type of starvation this causes and ordinary starvation. First, food and water [and, I would assume, a ring of sustenance or this ioun stone], do not negate this starvation as normal. Second, this type of starvation can't knock a creature unconscious or kill it.
I would allow it to work. There are arguments either way, though.