In my experience, foot chewing is usually allergies, but can also be a sign of under active thyroid. I would have the vet run a thyroid check. Often due to the constant licking and moisture, yeast can build up, making them more itchy and starting a vicious circle. You can try OTC athletes foot creams for that. To discourage her from licking it right off, try a good game of fetch or other distraction right after application. Also an omega 3 supplement such as salmon oil would be good, and also good for the allergic dog, if they're not already on something like that. Lastly, especially since it's a BC, it could be an OCD behavior. More exercise and/or a job to do could help, but there are also medications that help for OCD. I would consider a veterinary behavior specialist if all the medical efforts fail.