@Babydel
That's not correct, her ability works for everything. If an equipment give +50 of a stat, she would get -50 and vice-versa. If her sword had the description "This equipment regenerates your body", it would melt her body instead.
I can't give LN insight for this since the manga and LN are completely different in this fight, but from wikipedia's "Wound licking" page: "Wound licking is an instinctive response in humans and many other animals to lick an injury. Dogs, cats, small rodents, horses, and primates all lick wounds. Saliva contains tissue factor which promotes the blood clotting mechanism.". It may be the case that this creature has a heightened version of this behavior, and her ability causes her skin to rot instead of heal. This is only speculation though.