This is an Auto Immune condition that causes this with their nails and is quite common.
Simple things to do are give biotin tablets from superdrug or vitamin B complex tablets everyday to help strengthen and condition the nails.
Get boots for the paws to protect the quick when the nail comes off as you need to keep it clean to prevent infection.
Thank you George and morag give the tablets a try we do watch her toe and clean it when it falls of it's the same one lasts for bout year then comes off xx