At what point has be broken his promise?
Good point. IMHO if a person creates a debt that he is not going to pay for, then he is a scammer. There is a difference between "I owe you, let's settle my debt" and "I never promised to redeem it, so in fact I don't owe you anything".
Another flaw shown here (which I pointed out and asked about ages back - but it got ignored despite Ripple team answering easy questions in same thread) is that all IOUs are treated equally - just because I issue trust to A and B does NOT mean I value IOUs from them equally or am fine with IOUs from one being swapped to IOUs from the other without any input from myself.
Actually Ripple allows to specify the quality of debts on the protocol level, it is just not in the UI yet. Source:
https://github.com/rippleFoundation/ripple-client/issues/682#issuecomment-18076497