Unlike bacteria, cancer isn't spread from person to person, so it's impossible for the treatment of person A's cancer cells to impact the treatment of person B's cancer cells. However the cancer cells within in a single person can become resistant, which is why this treatment is a one time shot.

Resistance to what? There's no particular selective pressure on the virus.

If you mean the cancers becoming resistant, one of the appeals of using viruses in therapeutic settings is they can evolve right alongside their targets, helping mitigate some of those resistance problems.

You've got a good concern, just backwards: the virus is cleared from the patient, but the body becomes immune to the virus. Thus, it can only be used one time, which may limit the therapy's effectiveness.