Epistemic status: wild idea thrown out there in case it is brilliant.
Presumably, the blood of recovered patients has some antibody that targets the virus. Blood transfusions are widely understood and known safe. (Or at least safe so long as you don't do a few things that doctors know not to do, like giving AB blood to an O patient. ) Is it another thing that should work in principle, is more likely than not to work in practice, but by the time someone has run the medical trial, it will be all over. Of course, the recovered patients blood contains enough antibodys to make them immune, but I don't know how big an effect a fraction of the concentration would have. Of course given exponential growth rates, there might be far more critically ill patients than recovered patients. It might be possible to separate the antibody from the rest of the blood, or even grow it by infecting cells with covid19 in a lab.