Charlie is pretty much SOL and can't sue to recover from Alice because legally Alice doesn't owe Charlie anything. His sole remedy is to let Bob know what a jerk of a friend he has.
is just conjecture. A court of law may well decide that a Ripple IOU shall be an enforcable contract. That would be a distinct possibility today, and it will be more likely if people increasingly decide they want to use Ripple.
It's informed conjecture. A court of law may well decide that a Ripple IOU is not an enforceable contract, for reasons well established in law. The burden of proof is on the person attempting to the contract, and "the ripple ledger says so" is not going to overcome an alleged debtor's claim they've been hacked and never entered into any contract in the first place. As soon as Ripple has its first "allinvain" moment, and the ripple community asserts that that moment is an "irreversible" one, the legal community will quickly disagree and the world will understand why "the ripple ledger says so" means pretty much nothing legally, and that's when everyone will realize the screen door submarine is capable of no other destination than the seabed.