It is one thing to return an untested cat to a feral colony but it is a different kettle of fish to release a known positive infectious cat to an area where he will infect and kill other local cats.

If the rescuer wants this cat fine but this cat needs to be inside only for the rest of his life OR put to sleep with love, not to be left outside spreading diesease and getting sicker and sicker himself with no chance of survival.

With being a double positive, it is very unlikely he will live long than two years max and that is if he ends up living inside with full medical care, this is why are sweet Caramel had to be put to sleep, though he had advance mouth cancer too (my poor baby).

I know this decision is not yours to make but IMO this poor cat needs either a loving inside only home with lots of medical care or to go to the Rainbow Bridge now in order to protect the other local cats from suffering a similar fate.