It’s very difficult isn’t it. She definitely needs a friend and like people, if they are happier they can be healthier. One other thought but it depends on the rescue and you, but one or two rescues have a sanctuary or retirement run for guineas where they take on older or guineas suffering with an illness and cant be rehomed. I don’t know if there are any near you who do this. This way she gets company, but you would have to surrender her and check that they were happy to care for her. Another thought…