Girls, hi. My daughter started biting her nails. It started around two…

Girls, hi. My daughter started biting her nails. It started around two to three weeks ago when she was sick, and it's still going on. The nails are already like those of a gnawing rodent. Has anyone encountered something like this? How do you deal with it? How is it treated? Is it treatable? Explanations that it's not okay don't help.