If you want to keep her but are worried about having deformed babes then give her a few virgin females from you’re fry pool to hang with in a community tank.
I’ve never performed euthanasia, much less on a fish, but something called Oil Of Cloves is used as a fast acting topical numbing agent for toothaches so maybe it can be used for euthanasia. A drop on the fish to numb it’s whole body, then kill the brain with a needle or tip of a razor knife something along those lines sounds as close to humane as I can think of. Stick some on your gums to see how fast it works first. Might need to stick the fish back in a cup of water to wait for it to fully kick in and being cold blooded it might take slightly longer.
For now stick her in a hatchery box or something so she can’t breed.
Just noticed how old the topic is and added this to the post. What happened to the fish?