In this specific situation this is a huge problem because we ourselves have no way of linking a deposit to a Note, so unless we receive the Letter of Guarantee it's just a guessing game at best and we need to trust the user's word as we have nothing else to work with. We also have no way of verifying if the Note balance was partially or completely spent already because we do not know which one it is.
If you set a precedent by refunding this, and are expected to refund all future similar scenarios, then either you will go bankrupt or have to break that precedent when someone takes advantage of this and claims to still own a funded note which they have actually already spent. Better to not set the precedent at all, I would have thought.
We checked the databases and there is more than 1 Note with a very similar balance, we calculated using what the user recalled was his donation % used and the numbers do not match exactly.
Even if someone can tell you the exact balance on a note, this doesn't solve the situation. I could fund a note of 0.1 BTC, claim I lost the private key, have you refund me that amount, and then contact you again pretending to be a different person and the real owner of that note with a letter of guarantee to prove it.
In any other wallet or service if you do not save your seed phrase or private keys, then your coins are irretrievably lost. I don't see why Whirlwind should be any different.
Although there is obviously no way to guarantee that a user has actually saved the private key or letter of guarantee, perhaps you could not display the deposit address until the download LoG button has at least been clicked?