It is a bad deformity, but if she eats and swims about okay(ish, considering her deformity) then no need to euthanise.
But you don't want any of the deformed ones to breed. If you're still keeping all males and females together, then it could be a problem. If those adults produce a lot of deformed young, you don't want to continue breeding from them either, and it would better for the species overall if you euthanise any further batches they produce (since the females store sperm, separating them from the males isn't going to stop them producing more fry).
That's a lot to ask of someone, but you have to think of the health of the species overall. If you're giving or selling the other young with the slight bend in the spine, they're going to other people's tanks and being bred, and more and more deformed fish are entering the gene pool. We want to be responsible when breeding fish, and think of the quality of the species as a whole.
I have a male only tank and a female only tank, and have kept deformed females to live out their lives in the female only tank. Have a female guppy right now that has had a prolapsed bowel since she was born;
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She's smaller than other females from her batch, but otherwise seems healthy and active, so she'll live out her life with me, without being around males. I've kept the two other female guppies I produced that had kinked spines, without breeding them. If they'd gotten pregnant, like if I hadn't separated them from the males in time- assuming they even survived the birth - I would have euthanised the fry. Sounds brutal, and I'd hate to have to do it, but guppies have become increasingly weak because of poor breeding practices. They used to be considered the most hardy, easy, beginner fish, but they now have a reputation for being very poor, sickly, and die easily. Prone to deformities as well. People really have to be strict for the sake of the species, or platies will become as weak and prone to deformities as guppies.
