Human immunodeficiency virus targets t cells of the humoral branch of the immune system. The helper T cells are the "quarterbacks" of the immune mediated response. Without them, the body is unable to mount an effective defense against the simplest of organisms. HIV does not kill you. The opportunistic infection that forms when an individual is immunocompromised is what kills you. Advances in management of HIV over the last 20 years mean that the average person with HIV will live as long as an individual with Congestive Heart Failure or with Chronic Onbstructive Pulmonary Disease. As long as the individual is well medicated (and tolerated), viral loads are kept below detectable thresholds, and CD4 counts are normal, there should be no reason for an HIV patient to refrain from diving. Universal precautions are ALWAYS a good idea when helping an individual you are not familiar with.