3. Treat Whirlwind as a network and respect the rules, in this case we will not be able to help in any way and the user would lose access to his funds assuming it's true that indeed he lost the private key
Pretty easy choice, if you ask me. Option 3 it is. No keys, no coins.
That being said, even if you want to argue with some good will, I would recommend not setting a dangerous precedent just because you want to gain the trust of the community as a new service. That will go wrong in the long run and will be 100% abused in the future.
If the user didn't keep the private key, the public address or the letter of guarantee, and doesn't even remember the exact donation %, there is absolutely no reason for you to refund anything.