To the extent that the person has free will to choose to cause harm to his body, he can be culpable for sin. That freedom of the will can be compromised as a disease like bulimia progresses, so a person might not be as culpable later on for inducing vomiting or taking laxatives as he might have been during the initial stages. In any event, such a person would be well-advised to go to confession in any case. Whether or not the sin is mortal, the graces of confession (along with the proper medical treatment) can heal a person of the temptation to abuse his body.