What I got from the video is that they modify the virus so it won't cause disease and rather just modifies the host t-cells to recognize cancer. Does the virus remain in the host system? Can it still spread? If it spreads what happens to the newly infected person: cancer proof, nothing or HIV?
Hypothetically speaking here, if they put enough of the virus in her system it'd be able to act upon enough of her immune system to get those cancer-killing T cells spread around her entire body and kill enough of the cancer to stop it from coming back. At that point, maybe it's okay if all those T cells die off and her body doesn't know how to kill the leukemia anymore, because it's gone?
Cancer and remission are pretty complex, so I don't know. But that seems plausible to me.
I expect that leaving the virus able to actually spread would be harmful, even if it's been altered to target cancer cells. It would probably alter the immune system in other ways and potentially compromise it?
EDIT: According to another source I just looked at, the cancer-fighting T cells actually do replicate. So I'm not at all sure how they mitigate the downsides to that. Maybe this is a case of it being worth the downsides.
(Days ago, my friends lost their six-year-old daughter -- who looked very much like Emma in the video -- to leukaemia. So I'm very much in one of those "fuck cancer" rages right now...)