After confirming bud rot

If the other plant does not have any signs of rot no need to kill it. Do keep an eye on it though. Bud rot is more a localized problem in the plant and when it gets out of hand it can infect the whole thing. The molds fuzziness are its spore arms. so the more spores it releases the higher chance they will mold another part of the plant or another plant in the same area. Spores can survive for a long time and wait for the perfect conditions to grow. Low air movent, high RH and fluctuating temps. The spores are everywhere can't avoid it 100% but can lower some risk. One way to help avoid this is by keeping the RH low, and air flow up, this helps prevent the spores from germinating also air filtering but that's more an indoor thing. For me when I grow outdoors, I can not control the wet season near harvest, almost always 99% of the time my plant gets a bit of budrot or WPM in sept/oct. So what I do now is I only grow autoflowers outdoors and they finish up before the wet season starts in aug early sept and I avoid the issue outright. Other options are to grow big larfy sativa's, with lower bud density, this allows them to breath more and helps prevent the mold from finding a nice spot to germinate in. To tell if its rotten look for off smells, textures and colours. It will make it smell sour and gross, can turn it into mushy and slimy and the colours look like dead plants, brown, black, some yellowing. Good Luck!