To clarify, the modified HIV virus in this case was used as a vector to deliver an antibody specific to the cancer onto a T-cell. The T-cell then goes on to destroy the cancer. HIV is one of the most efficient vectors for genes.

http://en.wikipedia.org/wiki/Viral_vector

http://en.wikipedia.org/wiki/Lentivirus

http://en.wikipedia.org/wiki/Chimeric_antigen_receptor

http://www.nejm.org/doi/full/10.1056/NEJMoa1103849

EDIT: Also note that the HIV vector is used to modify the T-cell before it is inserted into the human. The HIV is never actually inserted into the human (in this case).