i am so very sorry to hear this, I hate seeing those 3 letters, and although it is a rare illness, I seem to be seeing them way too much at the moment. I would let the rescue group know, but you can't blame them, the majority of cats come into contact with the Corona Virus, and only 3% will get FIP, it is sadly a bad lottery. Good on you for wanting to help more cats though, do rescues in the US home over X MAs, I know they dont in the UK. RIP little ones.