(a) That's not right. A defendant is liable for damage of a foreseeable kind, even if some particular weakness or frailty of the particular plaintiff has exacerbated the harm that would have been suffered by a stronger person. This is known as the 'eggshell skull' rule.
In Dulieu v White & Sons [1901] 2 KB 669, Kennedy J explained the rule in these terms: "If a man is negligently run over or otherwise negligently injured in his body, it is no answer to the sufferer's claim for damages that he would have suffered less injury, or no injury at all, if he had not had an unusually thin skull or an unusually weak heart."