While it is a possibility that Willow's missing tooth is due to previous malnourishment, it is more plausible that Willow's malnourishment is due to her missing tooth - If malnourishment was the cause, I believe other dental problems would be evident.
Willow's missing tooth could be due to a number of reasons, including a congenital birth defect.
However, due to the lack of hair, Skinnies have a higher metabolic rate in order to maintain their body temperature, and are required to consume large volumes of food. Therefore, Willow's tooth may be preventing her from consuming an adequate amount of food. Therefore, Willow is at risk of developing further health complications, including, but not restricted to, respiratory infections.
Naturally, I would advise that Willow is examined by a cavy savvy veterinarian - Ideally, a veterinarian with experience of Skinnies -, to confirm whether Willow's tooth is missing; rule out further dental problems, and illness; and to discuss Willow's future health, e.g. dietary changes to accommodate for Willow's defect.
However, I would also advise that you weigh Willow every day for the foreseeable future.
Unfortunately, for Willow's health, should you feel that you are not able to meet her care requirements, I would encourage you to contact an individual who will be able to.
Above all, Willow is required to be kept in an environment with an ideal temperature, no draughts, and Willow should have a warm retreat at her disposal.
Please do not hesitate to ask further questions.
Best and warmest wishes.
which made me think she was neglected severely before she was given up. She did have some other issues when I got her too though like bumble-foot. It's been treated though 
