@ashleenicole For me if they are fighting to live on and are not showing any obvious signs of distress I let them be, and pass naturally. Although if she is simply sitting on the substrate unable to eat then it is apparent that she is under duress from her failing health.
If she matured at the end of May, nearly 7 months ago, she has lived a long life (egg-bound or not) for a mantid and is a elder female, and will only decline in health.
As such there is no need for her to suffer the final stages, either starving to death or succumbing to infection and related issues from sitting on the substrate (similar to bed sores in mistreated/neglected humans). Either of which would be much worse than simply passing in her sleep from freezing, and I would suggest freezing over starving any day (for myself included if it came to that).
Without opening up a whole can of worms/discussion on euthanasia, rather it is right or wrong compared to nature has no bearing on the matter, as it is a beloved pet and not somewhere on it's own in nature (likely frozen to death months ago from winter or found her end as prey long before that). There is no need for suffering, and will only shorten her lifespan a few days in this instance to perhaps a week at most.
With either instance though the quote "quality of life versus quantity" is very fitting and worth applying in this case as well.
Pain is one thing scientists have not been able to prove or deny in many wildlife species, especially when it comes to insects and of course mantids (
see the discussion and reference links). I've seen conditions in mantids that would kill a human or at the very least cause extreme uncontrollable pain, where the mantid seemed oblivious to it and goes on about it's task (such as a decapitated mating male for instance that can live on for several days without a head). Then in other instances I've seen a mantid react in a manner that could have only been sheer pain as a nerves or related condition could not replicate the complicated events and repetitively as witnessed (for example my first mantis Susanna that died in my hand naturally, near the end throwing her forearms around her head repetitively and very quickly like a human with a traumatic injury).