The monk in a game I am running was targeted by a psychic attack while she was asleep.  I made her  roll an intelligence save which she failed.  She wanted to reroll using Diamond sole which lets her spend a ki point to reroll failed saves.  I told he no because I do not believe she can use ki while she is unconscious. What does the community think.

 You let them make an Intelligence save.  Can you do that while sleeping?  It's pretty much a given that if someone takes damage they wake up, unless the damage is enough to kill them.  So once they wake up, they ought to be able to use Diamond Soul if they wish.

Nothing about ki or about the Diamond Soul feature requires the user to be awake, or even aware of the fact that they are making a save. They can definitely do it while asleep.

I'd say that checks out thematically, as well. Its called diamond soul, not diamond mind. Its the very essence of your being that helps you fight back against harmful effects.

I'd say that it all depends on how you look at what Ki is. Aethelwolf makes a good point that it's basically just your soul, your essence, fighting back against the psychic attack. It just happens to be that some of your Ki is spent doing so.

On the other hand, if you think that Ki is osmething that you actively use or, well, activate then you could argue that you'd have to be awake to use it. First  of we have the fact that the ability says that you "can spend" a Ki point to reroll the failed save, which shows that it's not something that automatically happens. A sleeping person can't take actions or reactions and while this ability technically isn't an action or reaction, it shows that you have to be awake to make conscious choices.

I guess it all comes down to how you have played Ki before and what you think it should be. Would it be a big deal if you allow it to happen automatically? Probably not.

I agree with Geann here. A save indicates some level of defense from the target. Diamond soul is enhancing that defense. If they are helpless while sleeping, they shouldn't get a save. If they do get a save, they should get to use DS. I understand though that activating DS is a choice, and making choices generally requires consciousness. But is it a character choice or a player choice? That's not really clear due to the vague descriptions of what a save actually represents. I think self-preservation is often instinctual, so even if it is a character choice, this might be a choice the subconscious could make.